Most logic puzzles (maybe all?) have a particular way of thinking about them that will lead you easily to the answer. If the puzzle seems hard, that's probably a sign you need to change how you're thinking about it.
Edit: Looking at it again, there's something else: Every logic puzzle is going to try to slip some important fact by you with vague language. In this case, the hint gives it away: the piles can be different sizes. In general, if I tell you to divide this pack of cards into two decks, I mean they should be equal, and you understand that. The puzzle is using that deliberately to trick you.
It's easy to get around, though; you just have to define the puzzle logically. A "deck", in logical terms, is an unordered set of any number of cards. Once you realize that, the answer seems stupidly obvious.