It's surprisingly decent if you pick songs that are already somewhat compatible. It's understandably terrible at deciding when to bring up vocals of either song (often having both playing at once) but I could see something like this working to help get some ideas if you already had something in mind.

For example, I put in two songs that I've actually mashed up manually. It wasn't nearly something you could consider "complete" or even "good" but it gives an idea of how it might work.