Actually it is. The way it works right now is any Magic weakness you have will affect further weakness potions applied to you. For example, if you have 100% weakness to Magic when you receive an 88% weakness to frost, the frost weakness will actually now be 176%. Similarly, if you have an existing weakness to Magic and receive a new independent weakness to Magic (in this case, Apprentice stone 100% weakness as one source and a potion as another source) then that new weakness to magic will be further enhanced and then added to your existing weakness. For example, with Apprentice stone weakness and then drinking a 59% weakness to magic potion, the new weakness will be 100 + 118 = 218%.
I have tested the damage to self, and in a bizarre twist it does not scale with the damage I am taking. I always take about 12 damage standing in my own blizzard, regardless. So I am puzzled as to how the Skyrim team put together the damage calculation for this spell.
Now try to poison a dagger with weakness with magic and get someone to hit you with it and then drink another potion
(silly enough to ponder why not just stack fortify restoration potions to begin with)
So that in the end, I think who deals more damage in the end is the one whom can attack the fastest.
Cause once you hit the max; you either crash the game from a number overflow;
or completely rely on damage formula's such that you have MAX/per second damage, or MAX x2 damage /per second, or 30x Max/ per second damage.