My 6yo DS and 2yr old DD share a room (as we only have one aircon in summer) and they love it. Never had a problme with one keeping the other up when they are sick - sometimes if DS has a bad cough, DD might stir a bit in her sleep but otherwise it is fine. We have a night light so I can deal with one without waking the other if there is a problem at night and generally they keep each other company. It probably does take longer to get them to sleep at first as they talk to each other etc but as I sit in with them until they go to sleep I feel it is fairer this way as otherwise I would be patting DD and DS would be alone IYKWIM.