Are you talking about snorkeling gear? I don't think you can rent SCUBA gear for personal use on
Castaway Cay, and even so, SCUBA diving alone is not really a smart idea. Assuming you're asking about snorkeling, then yes, you could probably rent one set and share it, but it's likely that a mask that fits you will not fit your husband, and vice-versa. Ditto for the fins, probably.
I'd suggest each picking up a cheap set and bringing it with you. You can get basic gear at places like Sports Chalet or Sports Authority for about what a rental would cost you. You're then covered for all future snorkeling endeavors.