FOOL,
I wouldn't even attempt doubling up. You will not get a proper bond between both sheets and they will flex at different rates, causing no end of issues.
You could try to get hold of 1/2" LAMINATED glass (2x 3/16" panes of float with a 1/8" thick plastic sheet bonded between them - known in the UK as 'bomb-proof glass') , but this will invariably be more expensive than 1/2" or 5/8" thick float glass. Much stronger, granted, but a lot heavier too.