The good news is that you don`t need to bond two disks together in the first place. Most telescope mirrors over 8" diameter are mounted in 9 point supports so they can be thinner - which costs less and weighs less.
A 12" diameter mirror only needs to be 1" thick when mounted with a 9 point support system. 1" thick glass is available for you. Some of the commercial suppliers of glass will sell you a disk. You should make sure it is annealed to it will not have internal stresses which would cause problems with a telescope mirror.
Bonding glass disks together has been done many times before, but often with bad results. It is possible to do this successfully, but you will have problems the way you are trying to do this.
First, the glass disks you are trying to use need to have equal thermal expansion properties. The best way to make sure of this is to get the glass from the same large sheet of glass. It is better to have only two glass disks in your composite.
The thickness of the cement is very critical, and each drop must be precisely the same thickness. The material used to bond the glass is also important. I`m not sure what properties are needed, but as I remember most will not work well enough. Many cements will stick to the glass OK, but most have some other problems. I think that the cements have a different expansion rate than the glass which means that the disks are going to be under stress most of the time. I wish I could remember better about this, but there must be some people who know more about this than I do.
Good luck if you do try this, but I advise you to stay away from bonding two disks together. You should make a mirror from a solid piece of glass first. If you want to experiment with cementing glass together, then I would do it as a long term experiment to see how it works out.