I'm reasonably sure (I think I did it once a couple of years ago) that you can claim on both where there is an outstanding balance. So, for example, if you have, say 70% coverage on your dental insurance for the procedure, you can still make a claim based on the outstanding amount. I don't think you can claim on both where you are completely covered by one of them (e.g. you have 100% coverage via the dental insurance). Either way, as Sue Ellen has said, use the snap and send for your health insurance, they'll very quickly tell you if you can't claim and it isn't a problem to ask!