Britt, first of all, just to confirm, you've got the flat sides of the stones together, not the grooved sides, right?
While a reading for 600 is highly encouraging, I think it's important to bear in mind that pizza bakes with stored heat, so in order to get the best possible results, the stone needs to be 600 all the way through, inside and out. When you have two stones stacked, it's going to take at least 90 minutes for the heat to travel to the center of the mass.
Have you had the single stone in this lower rack position before, and have you given it 90 minutes? If not, then I think it might be worth trying unstacked.
600 gives you more flexibility when it comes to stone shopping. As I said before, there's a good chance a high alumina kiln shelf might give you a 4 minute bake and you don't have to mess with the weight of steel.
Bake a pizza on this stacked setup with a... 100 minute convection preheat and then try a single stone with 90. Also, it won't be easy, but try, if possible to lift the fully pre-heated top stone and get a reading of the center area between the two stones. This should all be very useful data.