Assumptions: Using "float" or "plate" glass. Most public Aquariums use a safety factor of 3x or more. I use a safety factor of 2x when I am certain there will be no bending of the supporting structure.
Single window setup:
84" Length of glass window supporting water,
48" Vertical height of window supporting water to top of glass only,
.97" Thick with a Safety Factor 2x
1.19" Thick with a Safety Factor 3x
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Two window setup:
42" Length of glass window supporting water,
48" Vertical height of window supporting water to top of glass only,
.68" Thick with a Safety Factor 2x
.84" Thick with a Safety Factor 3x
http://anythingfish.com/Aquarium_glass_thickness_calculator.htm