I don't think it's too much to expect. We had our boys share a room from the time we moved into our house (younger one 6 m/o, older one 2 y/o), and they have never had a problem with it, aside from the occasional time that one or the other cried out in the middle of the night and woke the other up. Maybe it was because we started them out so young, that they were used to it.
I think parental expectations have a lot to do with things, though. Kids are notorious for testing boundaries, and seeing what they can get away with. Sometimes I wonder if parents' almost expectation that their kids will not do well in some way becomes a self-fulfilling prophecy somehow.
Anyhow, it may be best to remember Barney Fife, "Andy, we've got to *nip this thing in the bud*!" ;-) Your kids may need a little extra guidance and strictness in this area, esp. since they've been doing this for a while now. What I would try if I were in your shoes is as follows: Today, talk sternly to your older son about your expectations for bedtime [that he and his brother will be in their respective beds, and WILL STAY THERE; that he is to lie still in his bed and go to sleep, just like he did when he was in his room by himself; that he went to sleep so well, and was so good then, and that you know he can still do that]; perhaps also to your younger son, as much as you can on his level -- that bed is for night-night, and he will go to bed and stay there. Then, when you tuck them into bed for the night, you remind them again of your expectations ("remember -- you will stay in bed and go to sleep!"), turn off/down the light, and *almost* close the door [you may need to have the hall light off, so that they can't see you]. Then, if/when either of them starts to sit up or get out of bed, you sternly say through the crack in the door, "Lie down!" or "Go to sleep!" Very likely the child will be astonished that you saw him through the "closed" door, and will obey you. Then a few minutes later, he may try again, and you will still be there watching, so you will tell him again to lie down and go to sleep. As I say, "nip it in the bud" -- don't wait until the whispering becomes talking, or the talking yelling; don't wait until the child sitting up becomes the child on the floor, or over at his brother's bed -- reprimand at the first sign of mild disobedience (sitting up, whispering), so that it *doesn't* become disruptive.