I have a friend with a two year old who has joint custody with the mother. That has been the arrangement almost from birth; he and the mother were never in a traditional relationship and never lived together apart from a couple months at the beginning to get certain stuff down like breastfeeding/pumping. They get along and have been managing 50/50% custody reasonably well.
However, he's looking at moving to Africa—not immediately, but at some point. If that happens, co‑parenting will obviously not be as convenient as it is now. But he would have some living flexibility, which might at least in theory make things manageable given that he considers it important to still care for the child 50%.
I read a question elsewhere about doing 6 month intervals due to parents being in different cities, and the consensus was that it wasn't even remotely acceptable; but the main reason cited was having to switch schools mid‑year, every year.
My friend, though, would be able to do a different arrangement in which the child wouldn't need to move schools. Basically, he would take the child to his home in Africa for 3 months during the summer break, then mom would do 3 months, and then in the winter he would come back to the child's home city and have the kid there for another 3 months. (This assumes he can also keep a residence in this city, which seems likely.) So, basically, they would do 3/3/3/3 months, and the child would remain in the one city except during the summer break.
(He could choose any three months to come back, but winter seems to make sense so that mom doesn't go half a year without doing the primary care. Actually, winter isn't the best time for him to leave Africa, but it is what it is.)
The main issues I'd worry about would be:
For 9 months of the year, only one parent would be in the same country as the child, which would be stressful for everyone in the case of a serious emergency. (The trip is literally a day and probably can't be booked same‑day.)
3 months could be a long time for a parent and child to be away from each other.
Right now the three of them occasionally go places together locally. (They're not typical co‑parents in that there was never any failed relationship, so the dynamic is different.) This would only be able to happen within the 3 month window during which he's back in the home city, and that would probably be during winter, which is cold here.
The child wouldn't be around school/neighbourhood friends during the summer. (On the other hand, getting to vacation in a faraway place could be an enriching experience.)
The child would have three residences to adjust between: the mother's, the one in Africa, and the father's local place. Also, it's not entirely clear if the father's local place would be the same from one year to the next, and even if it is, it's possible someone else might live in it while he's away.
Could be harder for both parents to stay "on the same page" since they'd have no in‑person contact for 9 months at a time. (Perhaps technology could help to a degree.)
The main questions are:
Is this liable to do significant harm to the child?
Are there any other big hiccups in particular to pay attention to?
What aged children could typically handle this alright? He may be able to put it off for some time, but there are of course trade‑offs involved the longer he waits.