Having quite a bit of experience in the glass industry, I sleep well at night knowing I have no holes at the bottom of my tank.
Putting two pieces of glass together is even worse. Glass against Glass is very dangerous, unless you are going to put silicon in between, however the cost of the silicon is high.
15mm glass will be ok, costs more but much safer.
Plumbing could be a problem? The threads on the plumbing might not fit through the thickness of the glass.