Barry, if you put two stones in the oven at the same time, the bottom one will shield the top one and, in turn, get hotter faster.
When looking at stone placement, I'm not saying it's the case here, but there's a tendency for people to perceive stones like they do pans, ie, the closer they put the implement to the heat source the hotter it's going to get. What people fail to see, though, is that because stones are such poor conductors, during the bake, the bottom heat source has absolutely no impact on the bottom of pie. As far as the bottom of the pizza is concerned, the stone IS the heat source. If the pre-heated stone doesn't contain the necessary energy to fully bake the bottom of the pizza, then you're up a creek, because the heat coming from the bottom of the oven can't travel through the stone quickly enough to help. This is why people use thick stones. The bigger the stone, the more thermal mass it has, the more heat it can store.
A pizza bake involves two relatively independent processes. The stored heat in the stone is baking the bottom of the pie and the heat radiating from the broiler/top of the oven is baking the top. During the bake, you don't want to use the bake feature/bottom element, because whatever heat it gives off will never reach the pie in time. You do, though want to use the broiler while the pizza bakes, with the broiling element as hot/bright red as possible, and the pizza in close proximity to it.
As far as the stored heat in the stone goes, since heat rises, the stone will collect that heat wherever it is in the oven and it will max out based upon the oven's peak temp/whatever the thermostat cuts out at. Radiant heat is highly dependent on proximity, so the closer the stone is to the bottom element, the faster it will pre-heat, but the final temp/quantity of stored energy will be the same. The critical aspect of stone position is the heat baking the top of the pie. In an ideal scenario where you've got a great stone that can store enough heat and conduct that heat quickly to bake a quick pizza, you need to have the top of the pizza to bake quickly also- this is done by positioning the stone as close as possible to the broiling element.
Now, for ovens without a broiler, then that changes the equation a bit. In that instance, you might want to place the stone at the bottom of the oven and create a false ceiling (out of foil and/or a large enough pan) to collect the heat coming up from the bottom element and reflect it back on the pie. That's just for ovens without a broiler, though. A broiler will always give you more radiant heat than this type of workaround, so if you've got the broiler, put that stone at the top of the oven.
Achieving a faster pre-heat by putting the stone in the bottom of the oven can save time, but baking the top of the pie faster by putting the stone on a higher shelf/close to the broiler is far more important.