how does the 2 player battle tower work in Pokemon diamond?

you both pick two pokemon each. its a double battle where the opponent has two pokemon each also. same rules. i don't think there is a boss after 21 and 42 though there might be. hope i helped. any pokemon game makes a great gift because my cousin knew nothing about pokemon them i gave him the game for his birthday and know we play and sometimes he beats me now

You gain access to the tower either before or after you beat the Elite Four, I forget.

You two basically hook up and the game pitches you against AI controlled trainers, each one harder than the last. The battles are tag team, 6 vs 6 pokemon in total; 3 pokemon from you and 3 from your brother.

Have you considered waiting for the release of White and Black instead though? They're just a little more than a month down the road.