Do you know why your hubby is worried? Is he worried about losing you, or worried about your safety (physical or psychological) within your other relationship?
Must admit that whilst I'd be open to my wife having another, having another Dominant would likely be a concern for her well-being. Men are the "protectors" and that mantle is not easily discarded. And it doesn't have to be rational ... an eyes-wide-open D/s relationship can be much safer than straight vanilla sex.
I think it's cool that you're trying to be self-less. It seems your hubby is open to the idea, in theory. If I were him I would likely just need plenty of reassurance... us men are sometimes more sensitive than we let on.