Clerics (single, dual, OR multi) cannot use bows. Unfortunately, you're SOL.
You can always use Shadowkeeper to try to "fix" your mistake.
Dual classing is basically this: you advance in one class, and then STOP. You start as another class at level 1. When your new class' level exceeds that of your old one, you have access to that class' abilities. For instance, you start as fighter at level 9, then change to mage. You currently function identically to that of a single class mage except for your greatly increased hit points. Once your mage reaches level 10, you can use the figher's abilities, i.e., his weapons, armor, etc.
It should be noted that certain class combinations retain the penalties of one of their classes. While a Fighter/Mage can wear armor (the fighter aspect), he cannot cast spells while wearing it (the mage aspect). A Cleric/Fighter (or anything else with a Cleric) will never be able to use weapons that a Cleric is normally prohibited from using. That's why I don't see the appeal of having a dual/multi classed character with a Cleric; a Cleric is almost as strong as the Warriors and can wear the heaviest armor, but cannot use bladed or piercing weapons. The only reason I can see having a Fighter/Cleric is for the extra hit points from the Fighter, and possibly for some combat bonuses. But as I said before, a Cleric's a pretty decent fighter in the first place.
A Kensai is very skilled with melee weapons. The only problem is, he can't wear armor of any sort. The Mage also shares this weakness; this is why dual classed Kensai/Mages are so popular; they have a collective weakness. If you dual the Kensai at level 9, you can still reach level 17 with the Mage, the maximum level for a single classed Mage in BG2. The Kensai/Mage gets excellent fighting ability, 9d10 worth of hit points, AND he is a full-powered Mage. That's why people play this character.