I have done something similar to what you want to do. The biggest downside is I have to plug the headphone cable in when I want to use headphones as receivers mutes the speakers when the headphone jack is being used (no way around that). You could use a separate headphone amp but the other issue to consider is if you using analog pre-out's to feed a separate amp they will not output your digital inputs.
As far as using 2 headphones a splitter should work fine.
As far as using 2 headphones a splitter should work fine.