IANAL either, I think one goofy work around might be to have a game that is only an engine, and distributing only that, with instructions on how to use the assets from the other game. For example, I release game X, but it requires the assets of game Y. I don't bundle the assets, I merely tell the buyer of my game that they need to copy the assets of game Y into game X's asset folder, or prompt them to give a path to game Y's folder, whatever. The assumption is that the buyer of the game must own game Y. In reality players of your game may be pirating game Y to play your game, but at that point, that's not your fault.
This is sort of how Rifftrax works.