You can have two disks of glass fused together if there is a glass company near you that can handle the size. They will need a kiln which is big enough, and the knowledge to anneal the glass. You need to get two 40" disks which came from the same batch of glass - the same melt, otherwise the expansion rates may be slightly different.

Plate glass would work, if you are willing to use it. I have talked to glass companies about melting two glass disks together and they say it can be done. The best way may be to put several small glass spacers around the perimeter to space the glass disks apart. When the glass gets hot the top disk will sag in the center and the contact will progress outward. This will minimize bubbles.

A 2 inch thickness is a good minimum for your project, which would require a good flotation support, probably at least 27 points of support.
Even with a very thick mirror, you will need a good flotation support for it. You should check into that before you spend any money on a mirror blank. Here is a link to a website which can help you design a mirror support.

http://www.eecg.toronto.edu/~lewis/plop/

Good luck, let me know if I can be of any help to you, Steve