In a tag team match, when you have your special running, grapple your opponent (strong), and press L, R, A, & B simultaniously. You'll do your partner's special!
To easily unlock the Backlash arena, you must first have purchased the Armageddon arena. Then start a Pay-Per-View with the arena set to Armageddon. The Pay-Per-View should actually take place in the Backlash arena.