The topic here is to discuss "Difference between 50/50 Custody and Sharing Custody" in terms of (1) physical custody, (2) support money and (3) pros/cons of both and any other useful information.

Here are the differences with a warning from Mark Twain on the difference between lightning and lightning bug:
1.  Joint legal custody means that each parent has an equal say in the activities of the children.
2.  Primary residency means that the child resides more with one parent and the other parent has access rights.
     a.  Child support is based on the Guidelines with primary residential parent responsible for children's expenses.
     b.  Child support can be reduced if the parent not being the primary residential parent has substantial parenting time.
3.  Divided residency is when parents have 2 or more children and each parent has residency of one or more of them.
     a.  Child support is the difference between each parent's child support.
4.  Shared residency is when parents have  equal or nearly equal time with the children.
     a.  Child support is 1/2 of the difference between each parent's child support and the parents share (usually equally) the children's direct expenses.
     b.  An alternative to sharing the children's direct expenses is the Equal Parenting Time Formula with one of the parent's paying all the children's direct expenses.

Thus 50/50 with each parent having residential control of 2 or more children generates child support from the difference in child support while each child spending 1/2 of the child's time with each parent (another 50/50) generates child support from 1/2 of the difference in child support.

By djmlaw.

Guru:
Two points.
Be careful about what is in the statute and what is in the Guidelines.  The two are different but for better or worse, they both have an equal impact on the actual results.  The Guidelines discuss divided and shared (and shared with Equal Parenting Time & clothing agreement).
Second, Father's child support is $1,000 and Mother's is $800 for 2 children who are boy twins.  Father has residential custody of Cain and Mother of Able.  The child support from Father to Mother is $200 because this is divided custody.
A switch and now Cain and Able are paired up so each parent has both of them together at the same time or shared custody.  The child support from Father to Mother is $100.  Your question is why the difference?
The answer is that while in both the divided and shared situation, Father and Mother have the same hours with children.  In the divided case, each parent has all the time with 1 child.  In the shared case, each parent has 1/2 the time but has both children together during that time.
The child support in the shared case is reduced by 1/2 because when Father has both children, he will feed them steak and when Mother has both children, she will feed them hamburger-or Father's house is bigger than Mother's.  Mother gets less child support because Father buys steak for two boys every other sharing cycle so the wealth/poverty for the children is balanced out.  Father still has the same steak expense for the children in divided and shared but at least both boys get some steak and child support is for the children.
Incidentally, this is just my explanation to clients.  For all I know, it was a typo.
djmlaw.

Dear Guru:
I don't quite understand your example of the parents with equal income but a $1,000/$2,000 child support differential.  The real problem/solution is where the parents don't have the same income.  Consider Father noncustodial with 2 children who are twins in the middle age bracket and Father's child support income is $10,000 and Mother's is $1,000.00.  Father's child support payment to Mother is $2,021.69 and Mother's child support payment to Father is $202.17.
First, if you were one of the twins, whom would you want to live with.  I would pick Father.  In the worst case scenario, he is liable for only $2,021.69 in child support assuming that Mother initially has residential custody of the twins and it is going to take some time to see if the move of one of the children would work out.  Father has $10,000 minus $2,000 or $8,000 pretax for him and me while Mother has $1,000 plus $2,000 pretax for her and my twin.  Sure there are some tax issues but Father and I living on $4,000 a head compared to Mother and my twin living on $1,500 a head-guess where I pick.  In this example just looking at the standard child support of $2,021.69 to Mother with 2 children, Mother gets instant poverty along with the 2 children as $3,000 for a family of 3 is $1,000 a head and this is under the Guidelines child support.
The reason for Father paying Mother 20% of his gross which is 200% of her gross is so she can put hamburger on the table.
Now let's go to the divided custody which means that Father's child support is $2,021.69 minus $202.17 or $1,819.52.  First, I doubt that Father is going to have a victory dance.  He gets me full time for a $202.17 savings in child support?  That sucks.  In effect  he goes from $8,000 a head to $4,000 a head.  However, Mother goes from $1,000 a head to $1,500 a head.  She can now afford a better quality of hamburger for her and my twin.
Now to the shared custody where Father's child support is now $909.76.  What is different with the shared custody is that each child gets a shot at Father's higher income.  Let me do the steak/hamburger analogy again.  I live with Father and my twin lives with Mother in a divided custody arrangement.  I will get a better meal than my twin but Father's $1,819.52 child support will allow Mother to occasionally get some steak for her and my twin.
Now switch to shared custody.  Suddenly I don't get steak every night and my twin doesn't gets a good grade of hamburger every night.  When both of us are at Father's 1/2 the time, all three of us get even better steak because the child support decreased.  When both of us are at Mother's1/2 the time, all three of us get even a lower grade of hamburger because the child support decreased.  But both of us twins get the same food, it just really varies more between the homes when there is shared custody rather than divided custody.  But why shouldn't Father have the joy of watching us eat steak rather than pay Mother more child support so she gets that joy.  Remember child support is for the child and not to improve the custodial parent's life style.
Of course, you know that the poverty of Mother is just a fact of life.  You hit the nail on the head with the idea of spousal maintenance.  Interestingly, any spousal maintenance from Father decreased his child support income and thus his child support and any spousal maintenance to Mother increases her child support income and thus further reduces Father's child support so as spousal maintenance goes up, child support goes down.  It's just a matter of running the numbers to see the effect.
Finally, I just work here.  I have no idea what is the actual reasoning or if there is any behind these rules.
djmlaw.

Guru……

Your examples work for both sexes in reverse. Stereotyping is not helpful.

The real question is…. would use of the "family income" or "household income" vs. the individual parent's income be a better or more accurate representation of the economic circumstances of the child(ren)? Which is in my 360 degree view a HUGE can of worms for any one state to change toward. Leave it to the Federal Government, benefits system, tax system, etc. to set the definitions. Changing the model upon which the entree system of support for low income children is based may end up to be more expensive for everyone (both the parents  & the taxpayers) in the end  just like the effects of the health care system changes.

Guru……

Your examples work for both sexes in reverse. Stereotyping is not helpful.

The real question is…. would use of the "family income" or "household income" vs. the individual parent's income be a better or more accurate representation of the economic circumstances of the child(ren)? Which is in my 360 degree view a HUGE can of worms for any one state to change toward. Leave it to the Federal Government, benefits system, tax system, etc. to set the definitions. Changing the model upon which the entree system of support for low income children is based may end up to be more expensive for everyone (both the parents  & the taxpayers) in the end  just like the effects of the health care system changes.

KTM, What do you mean "sexes in reverse?"  What part of my statement do you think works just fine if we swap the roles?  Are we again talking about who typically receives child support?  If so, that's not a stereotype, that's a fact.  Mom's have primary residency and receive child support an overwhelming majority of the time.  I don't understand why you keep attacking that point.  It's just simpler to refer to payor and payee as dad and mom, respectively.

The reason I bring up household income is not because it's something I've considered, it's because if you read the surveys in 2011, you'd see that quite a few people bring up that point.  Hiding behind a spouses income is not fair to anyone including the kids.

I really don't meant to offend, but many attorneys I talk to refer to cases with "dad" paying "mom" or the other way around.  I suppose it's just lingo.  Just looking at the number of posts on this site alone I can say that most users are questioning what they pay, not what they receive.  So I suppose whether it's mom or dad, it seems like the paying parent is the one with the most questions.  The parent receiving support doesn't question the calculations, they question the other parent's hidden income.  Getting way off topic here tho.