It really depends on the cats. They probably *won't* adjust to being moved back and forth. You could try to do it the way that was suggested above and go to visit the cats, but I know that wouldn't do it for me. I also wouldn't be satisfied with one or two nights every so often.
How close are the cats? If they're not particularly close, you might consider splitting them up, or you may consider doing a more semi-permanent solution like two months or three months with each of you. I think this will still be frustrating to the cats, as they'll just be bonding with the person they're with and then will be moved to the other person and start to form a bond, only to be moved again. It will be less upsetting to them, though, than being in one place to go visit for a few days, in my opinion. Overall, though, I think that moving them back and forth is going to be very traumatic to the two of them.
If you do decide to share custody of them, make sure you do things like take a blanket that smells like them and move it with the cats. This will at least give them something that has a consistent smell to it. It will help them adjust a little. You might consider using more than one thing in that manner.. a blanket or bed or two that move with them, and maybe use a cat tree or something that never gets "washed" that will retain their scent. That being said, you might want to rub a sock on their faces (on the sides, mouth and cheek where the scent glands are) when they come back and put it on the cat tree, because their scents fade over time, and fairly quickly.
If they are more bonded with one person than the other, then they should stay with the person that they are most bonded to. If they are most bonded with each other, they shouldn't be split up. You might also work out a situation where you're either going to visit them regularly, or in such cases as your ex will need a "cat babysitter" you could go and take care of them, or watch them at your house while she's away, so that you get time with them. If you guys are friendly, then make time to go play with them, particularly if you can do it while she's at work if you guys are on a different schedule (and if she's okay with that and doesn't have a problem with you being in the house, or if they're living with you, vice versa)..
It's really a tough situation all around. I'm sorry to hear about your split, and most especially to know what kind of situation you're in with not getting quality time with your kitties.
I hope it all works out for you, and please keep us posted.