Kid Creole nailed it. All glass tanks, and even more so, all acrylic tanks should be supported all the way around the circumference of the tank. You might have no problem cantilevering 2 inches of the tank, but why take that chance if you can put a plywood support between the tank and the shelf to better support that portion of the tank?
Most glass tanks have the bottom glass siliconed inside the sides/ends of the tank, so the weight is born by the bottoms of the sides/ends, not the bottom glass. Then, most rimmed tanks have plastic frame which sticks out beyond the bottom glass, so all of the tank weight is on that frame, not the bottom glass. I interpret that to mean that typical tanks are designed to be supported solely by the plastic rim, all the way around. If your tank has the bottom glass attached to the bottoms of the sides/ends, I would interpret that to mean that the tank is designed to be supported by the bottom piece - so cantilevering 2 inches should work ok. (But, I wouldn't be willing to try it myself.)