From an article by Bodenhamer in a California Law Review article:
There are four criteria that parents must meet before obtaining a joint custody decree.
The first one is that it is a mutual proposal; almost never will a joint custody decree be had in a contested case.
Second, the parents of other custodies must demonstrate more maturity than normal. They both must demonstrate the emotional stability necessary to make joint custody work.
Third, they must geographically live close to each other. If the child is about to start, or has started, school they should live in the same school district.
Fourth, there must be a fair division of periods relating to events in the child’s life, such as birthday, school vacation, Christmas, etc.
On rare occasions the children are divided between the parents or other custodies.
Two factors contribute to these rulings. The age and desire of the children. If the children are becoming independent enough to cause the judge concern about their running away, and if they simply will not live together, then the judge will split them.