I guess its really a matter of personal preference and what you hope on getting out of the toons - I personally would say either go with a dps / healer combo or else two of the same classes - if you want the most potential in pve and pvp i'd roll a pally / priest run the pally as main with the healer on follow... (mainly because at 70 you would have a viable tank / healer classes if you ever wanted to pve as well as highly needed choices for any arena team) however if you mainly wanna go pvp i'd say two locks - if your rolling fresh go undead for wotf...

if you have a high lvl avail on a third account able to boost your toon (aoe lowbie instances) i'd def go that route (its how i lvl'd my pally/priest/mage x3 team from 10-60) if not then def quest them together