Have both your single classed cleric and mage focus on offensive spells, while your Mage/Cleric focus on healing/defensive spells.

Just my personal opinion ;)

Well its close to that....I use the mage and the mage/cleric for offensive and the cleric as my healer and as a 3rd melee guy....defense?? whats that?? lol ATTACK!!

Iam going with the them already. And believe me when i say that its lot of fun. I dualed the fighter on 3lv to mage and now although he is a whimp ,because he doesnt have decent spells, but its fun. I got 3 archer characters + 3 close combat. Nice. I just have run into some undead guy mythos, well he started casting somethning but he was already filled with arrows litteraly. As i have said its fun. Scouting ahead with thief, luring out enemies, slaughtering them ine by one. All those little aspects are what makes bg1,2,iw1,2 great.