IwubCheeze: By the time you get to Lower Dorn's Deep, your mages (assuming you're using any) will be able to cast summons making carrying around a single use summoning item pointless. I didn't need to use the item in Dragon's Eye but I just got sick of it taking up valuable inventory real estate and at that time, that was the best use for it..
It seems any party setup up can go through the game. I just finished the game with a solo fighter/mage/thief, it was really too easy later in the game, especially when I got the sunfire spell and ring of wizardry (the one that doubles the amount of level 5 spells you can memorize). Gonna try my "we don't need no stinkin magic" party with 5 fighters and a fighter/thief next, see how this plays out :D
Super fun fact: In a thorough run through of IWD. HOW and TOTLM, your party of 6 will get around 2700-2800 kills between them :)
Cullen12: I have two mages (as was the necessity in the BG series, and standard in traditional campaigns). They are non-specialist, but I have given one a fair amount of destructive offensive and summoning spells, and the other a fair amount of protective and defensive spells. That seemed like a good logical and traditional way to do it. I still think, though, that I might need to rely on that Bishop, as, even though my primary character is a Druid, I am sorely lacking in any Clerical capacity. Oh, I have the requisite heal spells, but very few other such incantations. It has just continually amazed me how this party that I assumed was weak, based on what I had read about some party templates for this, is able to hold it's own against most of the enemies that the game throws at you. I mean, I didn't realize that multi-classing was an option for non-human characters at all. I would have muli-classed my female elf mage. She's the one with all the defensive and protective spells. Still can really only used ranged weapons, though (which I guess is the point of mages in this game).
I think in my first playthough of IWD1, I had 2 mages as well but I specialized one of them (necromancer I think) and kept the other one a generalist because at that time, I really didn't understand how the restrictions would effect gameplay.
If your loading one mage with offensive spells and the other with defensive one, I think you'll find it better if you specialized them for an extra spell per level. Your offensive caster can be a necromancer. He misses out on enchantment and illusion spells which really aren't great anyways. Necros only miss out on 1 good spell, mirror image. However, if your necro is in the back with stone skin active, this won't be a problem. Also because IWD is undead heavy, I have never cast any enchantment spells other than sleep at the very beginning of the game. In BG, enchantmant spells were great, in IWD, they stink. Next play though, definitely consider a necromancer as an offensive caster.
Abjurers are also great specialist mages. Like the necromancer, they also miss out on illusion spells but like the necro, they can still cast stoneskin so it's not a problem. The other school they lose out on is alteration which has a few good spells worth noting but are mostly situational. Burning hands is an excellent back up for troll heavy areas and who doesn't like haste? However when haste wears off, you'll need to rest cos your party will be fatigued. This means you only need one casting of haste between rest periods. There's also one area in TOTLM where two castings of dimension door are essential if you want a really spiffy amulet for one of your casters. Some people like Tenser's transformation but I never used it cos I feel mages are for slinging spells and providing ranged backup when not casting. If I want a spell chucker to go toe to toe with the enemy, I'll use a fighter/mage with stoneskin and mirror image active. Most of these spells are situational and can be handled by the necro if need be.
If you like single class mages and don't care about making everyone invisible, I'm sure you'll find these two specialist mages much more to your liking than your two generalists.