a good thunk in the teeth and a firm "NO BITE!" will break the biting habit very quickly. i would imagine you are probably pretty entertaining to watch when you give lots of attention to the one who's been bitten, so make sure not to be too over dramatic with that.
a time out
every single time he hits is the only thing i know of that is effective in stopping the hitting. it might take a whole day (or even a couple of days if your child is particularly stubborn!) of putting him in time out, but at 19 months, he is very capable of learning the concept of hitting = timeout. when he hits, you tell him "NO HIT!" and put him straight into timeout (no warnings!) for a minute and a half. after timeout is over, you tell him, "you hit mommy and that is a NO NO!" and have him say sorry mommy (or whoever he hit).
these are the things that i did when i worked in daycare and they were always effective. good luck!