This is an issue people face with singletons, I know, but it's complicated by the fact that my twins are doing things differently from each other. And my older boy never slept in a crib at all, so I never went through this transition with him.
My guys are 24 months old, and one of them, Jake, refuses to stay in his crib. He just climbs out. Toby is perfectly content to stay in his crib, and in fact it often seems like he would love to just lay down and go to sleep, except his brother gets him worked up so he doesn't.
For a long time the routine had been to put them in their cribs, put Toby's blanket on him, and then sit in their room until they fell asleep. Usually there was a bit of chatter, maybe some requests for water, etc., but it didn't take too long.
Now I just can't get them to sleep! For some reason they've been better with naps (which I posted about before; if they are *really* tired, I can usually get them to lay down and nap in their cribs, these days, though the car nap thing is still a big problem).
I thought about converting Jake's crib to a toddler bed (I'll leave Toby in his crib as long as he's happy there), putting a gate on the door, and just letting him run around until he decides to sleep. That would work for Jake, but Toby still wants me to sit with him until he goes to sleep. More than that, Jake bugs Toby: he throws things in his crib, pulls his hair and ears, does funny things to make Toby laugh, etc. So neither of them end up sleeping.
(They won't sleep in my bed these days, even though they coslept until 12 months and still sometimes come to my bed in the early mornings. They won't fall asleep with me laying there. So that's not an option, unfortunately.)