Here's the part that got me in a moral dilemma: Because you are mixing two (or more) tilesets into a single tileset, you are passing the other's artwork as your own. Normally I think you would give credit for the artwork usage (Like in a report), but I'm wondering if getting the artist's permission is the proper way to do it.

Now I'm probably being a hippocrate because I am doing the very thing with two tilesets, but I fully intend to give them credit in both the tileset and the levelset they are going in.