Make sure to give both characters an equal amount of weapons, ammo, and healing items at all times, but if you're planning on giving some herbs to Billy, consider mixing them first as Rebecca, since he can't. Try to save your fire stuff for leech enemies, too. You should also store your items in a central place where you know you'll be coming back to.
I'm not quite sure how much damage you deal when you hit it anywhere other than the stomach, but I think you can hit it wherever and it'll do at least a bit of damage. The stomach is the weak point though.