I'll do chest and shoulders or back and shoulders, depending on what routine I'm on. I think chest and shoulders tend to compliment each other a little more.
I do legs and shoulders on the same day. I don't spend much time on my shoulders, only 2 exercises, as they are naturally a good body part for me. I also do traps (shrugs) on the days I do shoulders.
For me I try to group together body parts that have no interaction with one another. For instance:
legs-shoulders
Back - tris
Chest - bi's
this way I don't burn out a minor body part with the major....Just my own philosophy.
I do my shoulders either on their own day or with tris. It also depends on how much you kill them. I cant do them with a big muscle like back or chest, im usually to done. I switch every 3-4 weeks or so, ill do shoulders and tris, sometimes ill do shoulders alone and do tris with bis...all depends, personal preference pretty much.
I actually get to work-out a few more times a week than most people working at a gym so I only do traps with shoulders. But in a pinch I would imagine that shoulders and bi's would be day
I work out shoulders on a day on their own with some ab work and cardio before. Sometimes after a chest workout my front delts can be a bit sore so i leave a full rest day in between chest and shoulders.