Certainly. If you want the ability of the less dense 703 for grabbing shallow reflections but still want the extra density, mount the 703 exposed to the room and the 705 behind it.
I'll say again what I've said before though - for the price of 705, you can buy almost double the quantity of 703 - so for the price of 2" 703 and 2" 705, you can almost but 6" of 703 which will do a better job. If you already have the materials or don't have the room to do 6", the mixing is a good way to go.