Hana, I generally try to dissuade home bakers from stacking stones because, if the stones aren't perfectly flat, the heat won't flow from the bottom to the top stone fast enough and it will pretty much be just like using one stone.
Even perfectly flat tiles aren't going to be an ideal choice for pizza. At least not for the types you mention in your profile. It depends, to an extent, on where you are in the U.S., but Grimaldis is generally a coal oven fast baked pizza (and Neapolitan pizza is even faster). You can, in theory, use two layers of flat tiles, but in order to hit Grimaldi bake times, you're going to need some pretty extreme oven mods, such as hacking it to use the clean cycle. If you're not okay with extreme mods, then tiles are not going to be your answer.
Tell me a little bit about the new oven.
Inner dimension (width, depth)?
Peak temp on the dial?
Gas or electric?
If gas, is there a broiling burner in the main oven compartment?