I don't have any advice, but I just had to reply. I'm just sitting here now crossing my fingers since this is the first night my girl-girl twins are sleeping in toddler beds. They are 20 months old and one of them fell out of her crib last weekend. She's been continuing to lean over the rail all week so we decided to make the change today. I couldn't stand one more night (or one more minute!) of listening to her SCREAM and try to get out.
Anyway, we went right from the crib to the small toddler beds. I removed almost every toy from their room but left a few quiet toys in case they wake up before I do. (As if I'm going to get any sleep tonight!) I'm watching them on the monitor to make sure they are OK. So far, they've shared beds, switched beds, rolled around on their Dora couch, played with their dolls, ransacked the closet...and, oh yeah, banged on the door and cried three times. I don't want to jinx it, but it's actually better than I expected. (knock on wood)
Obviously at 20 months my girls are not yet potty trained. I also moved my oldest girl to a toddler bed at 18 months of age. For me, it was nice during potty training that she was able to get out of bed by herself in case she needed to go to the bathroom.
I didn't separate the twins now because I don't have another bedroom for that. But even if I did, I'd keep them together. They seem to calm each other down more than they do rev each other up. Are you separating the twins no matter what? If you're just doing it because you think your twins will be more disruptive to each other when they have more freedom, they might surprise you. But if you've already decided to separate, I vote for separating and changing beds at the same time. It might be harder to make 2 transitions rather than just one.
Good luck and let us know how it turns out!