Other than common sense limits, there aren't any. The problem with using Barb with a Cleric build is armor. Barbs can't use heavy armor, but clerics can. If you multiclass the two, one will lose out on class benefits. You can offset this a bit with a higher dex score though. If I were to dual a cleric, I'd go fighter. I think I'd take a few more levels than just four though, maybe 10 or so. Prior to char. level 20, fighters gain 1 point to their AB per level, so if you're looking for a bit more combat skill, that's the way to go.
Clerics, however, tend to be quite efficient as melee fighters. A few good spells, and you'd think you were one. Dualing to Paladin is also a good idea, if a little limiting, alignment wise. You may want to try a single class cleric, but make what I refer to as an oops save right before you'd want to multiclass, just in case you decide you don't like it, that way you can go back, and take the other class, if you really want to.
Another good possibility would be Champion of Torm. The classes compliment eachother, and you would gain the benefit of lay on hands for some quick heat of battle healing, remember that that is level based though. Lots of good possibilities, rogue, for example. The best advice I can give is to feel it out, and see what really works best for you.
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