My boys are in different rooms but across the hall from each other. Mostly they don't disturb each other but sometimes Tom (baby) will wake Jack (toddler). Jack will go straight back down though so I don't see it as a huge problem. The main thing is that I tend to go into Tom pretty quickly when I hear him because I worry about him waking Jack. I need to do that less I think as it's probably contributing to Tom waking more often.

I think the earlier you do it, the easier it will be for them. As I say, my boys rarely wake each other because each is so used to the other making noise, they don't even notice it enough to wake. I guess it's a bit like not hearing trains or planes anymore if you live near a station or under a flight path - you just tune it out.