Why wouldn't you declare which dice are and are not? Maybe this is completely unrelated, but I think of the case of the signum in devastator squads. One model is +1BS, but fires at the same time. You declare which die will be BS5 versus the rest at BS4.
Yeah, it's pretty easy. Either "Here's my 10 shots with tank hunter (roll, roll, roll), and here's my 20 shots without (roll, roll, roll)" OR "Hey, these 10 green dice have tank hunter, these 20 red dice do not ..." Pretty simple really ...