Also, if making one of them a tank, which talent tree should I use? I guess a shield is a must have then, so will e.g. enchantment still be good even when not dual-wielding?
The second you run 5 of the same you conclude you wish to specilize a guy ....


This is exactly why the optimal 5 man group is:

Tank: (Pal, War, Druid(?) or DK)
Healer: (Sham or Priest)
3 DPS: (Mage, Hunter, Warlock etc.)

I would go Pal, 3 mage, Sham but other combos are also good.