Twice a week for a bodypart is fine. All muscle groups as you have stated need at least 48hours before you hit them again. The key to know is when to hard core train and when to stop so your not over training.
not according to Big A's Growth Principle's, here is an excerpt relating to this:
"A muscle will not grow until it's recovered. The
muscle will not begin to recover until the nervous
system is recovered. It takes roughly 24hours for the
nervous system to recover from a workout. Only then
will the muscle begin to recover and grow. So, you
should never train 2 days in a row. Even if you train
different bodyparts, you still use the same nervous
system. You train 2 days in a row, your nervous system
recovers, but by the time the muscles begin to, you
train again, so the body has to concentrate again on
recovering the nervous system.
A training frequency of 3 days per week (Mon, Wed,
Fri) is more than enough. Numerous pros, including
myself, train like this offseason for maximum growth.
Even if you use streroids, you still have to train
like this. Steroids increase your recovery ability,
but they also make you stronger at a quicker rate. The
extra strength will give you the ability to train
harder/tear more muscle tissue, so you will need the
extra recovery that the steroids will give you."
-taken from "Growth principles from Big A"