First off mama, I want to just give you a big ole' hug. DS2 was an awful sleeper and while waking up once in the middle of the night seems like a lot, IMO an 18 month old will ideally be sleeping through the night entirely. (Notice I said ideally...lol) So I completely understand where you are coming from. My first POA is to talk to hubby and see what he has in mind as a resolution. I think him being in your room may be harder for YOU than anyone. At 38 weeks pregnant, you sleep like crap anyway so any stirring, etc is bound to wake you. I would try to transition him to his own room so at least you aren't stirring until he is obviously crying. But can I also say, that DH better be getting his butt up at either/or one of those 2/6 a.m. crying fests! Working or not, DH is expected to 1/2 the nightly routine around here. :-x
I would try to cut the milk and do water only. Have you tried a sound machine? Maybe it can help him to sleep through the night. I know our P&P made a scratchy sound when DS moved so we eventually moved him out b/c it would wake him up. I HATED that stupid p&P!!! I don't know if they all have that same material or not though.
I agree that the routine doesn't necessarily HAVE to change but I can't imagine waking up now at 2 and 6...then when new baby comes waking up every 2 hours on top of that. For your sanity, I think it should change. ((HUGS))