Roland + Axton = Turrets combine to form floating gunship with two layers of shields that drops Christmas trees which give out a healing/ammo aura.
Lilith + Maya = Maya uses phase lock to open a portal that Lilith phase walks through to fetch an dimensional abomination to fight for them.
Mordecai + Zer0 = Zer0 attacks with his Katana while riding a giant Blood Wing and they're both invisible. Attack starts with Zer0 and Mordecai spouting a haiku about enemies being, "so big, so angry, so dead." --- http://img339.imageshack.us/img339/4489/actuallyworks.gif