It may be an old wife's tale, but I was told as a child not to hit two hammers together as one might explode.
I'm not sure how it happened, a friend of mine had to have a piece of hammer head removed from his arm when he was an apprentice.
I don't think you'd be able to close a rivet with a dead blow hammer Ron, as they are normally plastic.
Though I think you can also buy hard faced ones.