I found a 10oz mag citrate bottle too much for my body and i'm 180lbs, so i wouldn't want to add Epsom Salt on top of it. Mind you that my wife is 130lbs and a 10oz bottle was just right for her. It all depends on how much magnesium one's body already has when doing the flush, so you have to experiment for yourself and adjust the dosage based on what works for you.
Next time, i'm going to take quarter of the 10oz. mag citrate 2-3 hours before the flush and the rest during the flush.
So the answer is i don't know but myself, would not mix them up, and would stick with either or.