My two have always shared a room and have done plenty of sleep training with them both in the room. Is your oldest a light sleeper? There were so many times I was sure the little one crying would for sure wake her brother, but actually she rarely did and we went through several phases of sleep training. We bought the oldest a set of kids ear defenders (about £7 on eBay if I remember right) and if he did wake up he would put on his ear defenders and go back to sleep. On the one or two very rare occasions he was woken right up, I'd sent him to go and get into my bed, and transfer him back to his bed once the baby was back to sleep.