Okay, I don't have twins, but I do have a 1.5, 2.5 and 4.5 yr old, so I understand the issue. My 2 and 4 yr old sleep in the same room, and we are planning on putting our one yr old in with them once she is done teething...
When we took our 2 yr old out of her crib, we had the same issue with her and her older sister. It took a few weeks to get them used to the new arrangement and several cranky nights and days, but eventually they figured out how to sleep together without waking each other up. We just had to be firm and patient.
One thing that works for us when they decide it is play time instead of bed time (they ARE kids after all), is we put them in their "chairs" (actually they are the chairs they sit in at meal time), and situate them so they cannot see each other. For a while they would wail, but we told them we would not take them down (there are straps on the booster seats, so they can't get out of the chairs) until they learned how to be quiet. We would wait it out until they stopped crying and stopped talking and were completely quiet. Then we would put them back in bed. And lo and behold, they would go almost straight to sleep. I think it is because they had gotten completely calmed down while in the chairs and the sleepiness set in.
They have learned now to be completely quiet when put in the chairs (this still happens every so often...it seems to go in cycles) and they are usually only in them for a minute or so and now go right to bed afterwards without a peep.
We only give them one warning (if we hear you playing, you go into your chairs). Before, we tried the "choice" option (chairs or bed?), but my 2 yr old couldn't quite figure that one out, so we stopped that and now it is just one warning and "plop" in the chairs they go. It works better that way and our 2 yr old no longer is confused about it and thus no longer wails.
We do not yell or lecture (at least we try not to), all we do is wait for them to be quiet with the statement, "When you are quiet, we will let you down and you can go to bed." That is all we say to them.
Oh, and when they go back to bed, we make sure we give them hugs and kisses and tell them how much we love them.
Hope this helps.
K.