Oh how I have felt your pain. I like in a small apartment and we don't have the space for the kids to have separate bedrooms. So my 4 and 3 year olds share a room. Mind you, they are 14 months apart, so it can be *fun* sometimes. They used to have beds next to each other, but they kept playing and crawling in and out of bed, so finally I moved them across from each other and for some reason, that keeps them separated enough at night so they don't climb in bed with each other, but together enough that they still know that "sibling" is there and isn't pulled away from them. My kids got used to sleeping in the same room, so when I would try to take them away from each other at night (my son sleeping on the couch, my daughter in the bedroom), it resulted in one kid waking up 500 times a night. My kids will sometimes play at night, but they stopped that once I sat in the room with them (not saying a single word, not looking at them, just sitting there) and if they got out of bed, I put them back into it. Eventually (after about 3 solid weeks of this - sometimes for 3 hours, sometimes for 15 minutes of me sitting there) they got the point and it was a month well spent for me because now they go to bed peacefully and I barely ever have an issue with them. I started in the middle of the room and then I gradually (one week at a time) made three steps to the door and at the last week, I was sitting in the door frame and essentially it was for 5 minutes only and then they would fall asleep and I'd close the door.
If that method doesn't work, you might want to consider separating them if they don't get upset with it.
Also, you might consider rearranging the room so that they have less to "work with" at night. Put all their toys are in a rubbermaid tub you can pull out of the room when they go to bed so they have nothing to play with. My kids have a table and chairs that we take out of their room at night because otherwise they will play with it. Take away the temptation for them to be naughty. It's worth the time to figure out what works, otherwise you will be pulling your hair out. Good Luck! And let us know how things turn out!
Beka