I did a search and don't see a lot of people using an upper and lower stone in a home oven. I have an older electric wall oven that does ok with stone on lowest rack. I use a super peel so can not put the upper one too close but was wondering if would help or make the pies cook slower with two.
I use two stones in my home gas oven with very good results for a few years now. My bottom stone has sides, so I purchased a new stone that sits directly on top of the sides and creates a box configuration. I do not move the pizza to the other stone part way through cooking, but i do turn the pie midway through the 7-8 minute bake time.
A mistake I made when purchasing the second stone was not paying attention to the depth. It is not as deep as the bottom stone and in turn does not cover the entire top of the pie since not being as deep as the bottom stone and why I have to turn midway.
The result of having the second stone are worth the extra few bucks, the tops of my pie turn out way better and eliminates the need to finish a pie under a broiler like some here on PMF do. If I forget to turn the pie, the proof that two stones are better becomes very apparent, because the side sticking out beyond the upper aren't nearly as browned as the rest.
Just my $.02