Other than appearance I don't think there is any difference whether you are using two of the same type of weapon or mixed for dual wielding. I used whatever I found that had the highest damage (though with low levels of the dual wield skill, I would tend to go more for bonuses than damage for the off-hand weapon, since the relatively high penalty would effectively even out the damage anyway).
Weapons can either do normal damage, or magic based damage. Magical weapon damage is independent of magic spells, and neither influence each other directly.
Melee damage gets a bonus from strength, ranged damage from dexterity (both 0.5% per point) and magical weapon damage gets a bonus from intelligence (1% per point).
Since it is fairly easy to find weapons with mixed damage types, or predominantly magic based, some people feel intelligence is the best primary stat. I focused on strength and conventional damage though, and didn't have any serious problems. The game is biased towards magic, though. Balanced and magic weapons are more common than conventional weapons, mana bonuses are higher and more common than hit point bonuses, etc.
In theory a mixed damage type on a weapon would do more damage on average, because some opponents are going to be weaker against one of the types than the other. In practice there isn't a huge difference in resistances, so you can pick one or both and do fine. If ghosts could only be hurt by magical damage, or there were stone trolls that couldn't be hurt by slashing weapons, just blunt force, then you might need to switch your weapons and/or tactics by region. In this case, magical damage does get more of a stat bonus, but other than that melee, ranged and magical damage are all pretty comparable.
