On Adv2, the left button is programmed as a momentary keypad shift button. On Adv1, which is running a TMK via adapter, it is programmed as a layer shift button. The right button is programmed as a shift key in both boards. Both buttons function as any other key on the board so they are fully programmable through the on-board programming.
As for having enough 2u keys, for each board, you need two full (104+ keys) sets of keys caps. Two each of numpad enter (for Backspace and Space) and numpad 0 (for Enter and Delete) keycaps are needed for the thumb clusters. Unless, of course, you can source extra numpad keycaps - I had no luck, at least not in decent quality blank ones.