You knit together whatever stitches are supposed to be bound off for the shoulder, as if you sewed them together. Any collar stitches are picked up the same way as you would have if you’d sewn them.
The only complication is if your shoulder had a lot of shaping to it. I often decide that I can live without the shaping, or do some short-rows to accommodate it. If you have a lot of shaping on the shoulder to have them slant down from the neck to shoulder, I’d be happy to try to explain how to do the short rows. If it’s straight across, then it’s easier than seaming, honestly.
The only real difference is that your seam will be magnificent. :teehee:
ETA–Here is a picture of a sweater whose shoulders were knit together. In fact, I just today finished this sweater and it’s drying.