ive read somewhere, cant remember where, but that as long as the genus is different (first name in the scientific name) then it should be ok. Dont have any first hand experience with double groupers, I know my blueline is a mean sob and I wont be able to add anything else into the tank with him in there, hes already killed a trigger, and im not wasting anymore money on fish. If you add them at the same time, it might be ok, as they are territorial, mines basically took over my tank, hes in there with a volitan. Personally id prob stick with one, cuz if there is an issue with two, they are hard to catch because they chill in the rocks.