That is a great idea, but stacking another on top full of water would make it very top heavy and the fact that they have taken about 25% of the structural integrity out by having the opening for the door would also be a factor. I still like the concept.
I"m guessing there's attachments for a hose for water - but I like the idea of a small tank on top for holding say, 15-20 gallons of water that can be heated thru a solar heater, or just allowed to warm in the sun. Wouldn't work up north very many days, but in SE Texas you'd have warm / hot water most of the year just from the warming effect of the sun on the tank.
Looks like a great addition to someone's private camp.
The only negative I can see is that those white steel frame polyethylene (polypropylene?) tanks tend to sun degrade pretty fast when they dont have water in them.