If you have such a weapon and land in a battle with two enemies that don't join, does spending a charge for the first battle last through the second, or do you have to spend a second charge? It seems like the rules say you can only spend one charge per turn, so perhaps the answer is neither? What do folks think?

I'd say it only applies to one battle, so pick your battle carefully.

I agree with Dam. Battle is separate for those monsters regardless of shared attribute.