Glass table supporting 12 gallon aquarium
According to the tabletop glass load calculator found here, and to my slight surprise, this table top can hold about 400 pounds.

I certainly wouldn't jump up and down on it, and any scratches might compromise it to some extent.

I told the calculator the glass is tempered, 1/2" thick, 18" x 54" with supports spaced 4 feet apart. Make sure you know the glass is tempered, not annealed. If it's annealed glass, it'll hold about 100 pounds.

That 1/4" sheet you just ordered would only hold 54 pounds on its own if you swapped the sheets. If you lay it on top of the other sheet, I don't know how to calculate how much stronger that will make the table, although it certainly will add to the load the other sheet is holding up. Maybe somebody else has some expertise in the benefits (if any) of gluing (laminating) two sheets of glass like that together.