Ditto on the 3 months. You also need to keep an eye on them to make sure there is no fighting. If you see one keeping the other one from eating you must separate them.
I agree with Scaley. If they can't spend the cash for a cage, what else will they try to get off cheaply with? Well they say, "Oh, that plant costs too much, I'll get that cactus since it's only 2$!" They'd just end up killing the cham later on anyways.