I've heard talk of some sort of bracers that doubles critical damage. Does it work with bows are is it just for one-handed?

The only similar thing I can think of right now is the Dark Brotherhood gloves, which only double backstab (sneak attack) damage and nothing else.

They only work for daggers + one-handed weapons, not bows.

Backstab gloves are mainly useful to make sneak attacks with daggers extremely deadly (especially in combination with the assassins blade perk). They do nothing for bows, but to be fair you don't really need such a bonus for bows. High sneak + archery = instant win