Making Cube out of 2 x 33G?

[sphelps]

If you keep the tank below 20" tall and do not increase the the length (36") your tank will be fine provided you brace it.

Actual volume has nothing to do with glass thickness, only height increases pressure (m*g*h), however the hydrostatic force created by this pressure will create a bending moment (deflection) and the longer the tank the larger the bending moment, but 36" is fine for 6mm.

No point going overkill on glass thickness, you just loose clarity and add weight.