There is no double sided print support at the moment in Stackz. This feature might be added at some point in time, but it has hardly ever been requested so it's very low priority.
The "print as flashcards" function has still its sense even the single sided way it is today; for exactly the purpose that you mentioned - you can print out the words that have been accumulated on the left side of a Stackz document as cards to carry them with you, arrange them on the desk, sort out the easy ones etc... things you can't do with a printed list. Really learning using the printed cards involves covering them partly, which is not ideal of course, but possible.