2.66 Ohms in parallel. 12 Ohms in series. Either way, not a great idea. And to keep this short I'll just say that the two cabs will not be operating at the same volume. So one is not contributing that much to your volume. If I were you I would just run the 4-10. You won't be getting much out of the 15 in your case. And, yes, you risk damaging your amp. With the 4-10, everything is running in the range for which it was designed, with the exception of watts. However, if you are careful, and listen for signs of speaker distress, this won't be a problem either.
Short version: Just run the 4-10 OR the 1-15. You will get more volume from the 4-10. In either case, don't peg the volume. Just listen to the speakers. They will tell you when they are getting all they can take.