I recently grinned to buy Armamentarium from Xur, and spent 4 hours killing and doing patrols in the plaguelands to get my Memory of Felwinter, and was saddened when their affects don't stack, and I think it would be fair if they did stack, since I did give up my super.
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I would assume it wouldn't stack, as no abilities in destiny previously let you have more than two grenades/melees. You could stack try to stack abilities like gift of the sun and Starfire protocol, but you could still only have 2 fusion grenades. Also, with gamblers dagger and the sealed ahamkaras grasps, you still can't get 3 three throwing knives. Kind of disappointed, but it doesn't surprise me