I'll spell it out because first few attempts didn't go through:

if we just say "roll any faction and we'll fight" what will ensure we don't end up with say 80% hordes and 20% alliance thus making the fight less interesting if imbalanced

maybe we can say... if your dualboxing nick name starts with a-h do horde and i-z do alliance ?

(that's not an equal split - maybe forum admins can post the distribution of nicks - or we could do sha1(nick)%2 = 0 => horde = 1 => alliance but that may be a bit too tough for ppl)