I have a MK1 GP2X, a Cradle and 1 AC Adaptor.
I plug the AC adaptor into the GP2X, then the GP2X into the cradle. I then plug two USB joypads into the cradle.
My brother and I have played many two player games together with this set up, no problems.
SO the simple answer is - if you arnt using anything more power hungry than two USB joypads you only need one AC Adaptor.