I second what Camaxtli says. It is extremely difficult to practice two specialties and maintain adequate skills. Part of being a physician is being a lifelong learner, and that means consistent exposure to a variety of cases that challenge you/hone your skills. That said, if you like rural practice and literally plan on being out on the Yukon or in a tiny town in Idaho, that would be an awesome option. Or if you plan on employing physician assistants in a private practice and then run or consult for a dentistry office, that could be a very lucrative option. Again, difficult to do both.
There are a ton of mixed residencies out there. Medicine/Pediatrics is very popular, so is Medicine/Anesthesia. In the end though, if you look at most of those physicians, most of them are primarily only practicing one specialty. The exception is rural practice or those in the mission field. 🙂