Aragonite helps to buffer the PH
in your water to keep it at a satisfactory level, but salt is a buffer as well, so I can't imagine it can make that huge of a difference (but, keep in mind, I haven't used this silica sand before so I'm kind of talking without real backup).
I'm putting down a 4-5 inch sandbed. I'm going to do what you said - lay down 2 inches, start the cycle then if southdown becomes available, throw it on top. I would assume, however, that you would need a couple inches of the aragonite sand on the top to play it safe. Either way, its going to get mixed in by fish and inverts. Maybe someone else on this forum that has used Quickrete or other silica based sand can chime in with real world experience.