Well u can always hold back in your hate generation, if u guys decide that one will be main and the other will off tank just don't use the hate generation abilities so often, fights can go really wrong if the tanks start to fight over who will be main and who will be offtank. Also IMHO main tanking is usually less demanding than off tank in 8 man dungeons, so if u consider yourself better geared u should probably go for the offtank position.