They hit two different target areas, so it isn't like they are interchangable brother in my opinion. Squats hit the Quads primarily. Deads--the Spinal Erectors. Both hit the Hammies hard as a secondary muscle group however, but that doesn't mean you can switch one for the other.
If your lower back is giving you trouble from doing both of them, then just try spacing them a little further apart. Personally, if I were going heavy on them(which I don't anymore, although I still do them light), I wouldn't be doing either more than once per month. Since I go light on both though, I do them every Quad & Back workout.