At creation, I tend to assign all my characters exactly two main responsibilities from the list of: fighter, archer, mage, priest, lockpicker. Pair off those roles however you like, and you should end up with a reasonably effective party.
Two Mage/Priests at the back is a favorite of mine. Specialize each one only slightly, and unless you're casting high-level spells, you don't have to worry about which is which. Very handy if one gets disabled.