Re:Arms, twice a week
If you think that you are training your arms once a week already, then you are probably already training them twice or even three times.
When you train your back with any type of pulls or rows then you are hitting your biceps a second time. If you are training chest or shoulders with presses (almost inevitable) then you are training triceps a second or even third time.
This will only just allow your arms to recover, but you might get your biceps in a second time and still recover. As I said before give it a try and see how you get on.