I'm going to have a 20 month gap between ds 2 + 3 in 3 weeks!!. I already have ds 1 who's 5.
Ds 1 + 2 sleep from 6pm - 6/7am
I am going to co-sleep with ds 3 to help me get as much sleep as possible. We did this with ds 2 and have a positive experience. It helped with b/f. I am going to use a sling (hugabug) with ds 3 so I am mobile to play with ds 2. I had severed PND with ds 1 and know that for me sleep is a key factor in my well being. We will introduce ds 3 to a bottle at 10pm as we did ds 2 in order for me to go to bed at 8pm (dh gets in at 7.30) and get some decent sleep till the 2am (ish) feed. This seems very regimental but it worked wonders for us and was only for a few weeks which compared to PND was fine. Dh is very supportive and we did talk about our plans and ideas before ds 2 arrived and I can honestly say we really enjoyed him and felt we were much better, calmer and happy parents to him.
Ds 2 isn't walking (no physical problem just very lazy ) so I do know that's going to be hard, it's hard now being 36 weeks pregnant (I'm having a c-section so are due a week earlier)
I am worried about b/f though as ds 1 got not jealous as such but didn't like it when I fed ds 2 and would go for a poo at that time, bounce on the sofa next to me, try to hug me round my neck etc etc. I tried the box of special books/toys but he didn't give a monkeys so I'm a little worried how it's going to work with a non-walking 20 month old. Also the summer hoildays will be tough but I will have help so I intend to make full use of that too!
A close friend has an 16 month gap and she has told me it's been very tough and getting out to chat to other mums has been the hardest bit as she's said she's been very lonely at times. I worry that b/f will mean I won't be going out much too...
I think the key thing is to talk about how you feel, how you're finding things and work with dh/p so you're both getting as much sleep as possible!