I am sure a glass tank would support its own weight, and then some, placed on top of it. What I cannot understand is why you would even consider stacking them one on top of the other. You need regular access to both - at least 3 times/week to provide fresh water (which you should be doing), as well as feeding and cleaning. A “tank” can only be accessed through the top of the unit, so that would mean moving an occupied tank on a regular basis to access the one below. Commonsense should dictate that this is not a good thing for a number of reasons. You might not mind the exercises but it will likely stress the occupant, slop water from the water bowl, and greatly increases the chance of breaking the tank. Just one sharp, unintended bump against a solid object is all it takes to break glass.
One needs to look at it in terms of what the snake/s require, rather than what might better suit you, and it then becomes a no-brainer. You clearly need a frame.
