The sides shouldn't be tempered. You could connect them with a 6" clear PVC pipe tunnel!, Get some big time bulk heads, and find a 6" hole saw!
At the very least, you could set them next to each other in whatever config you like (L shaped, end to end, side by side), raise one up 6 inches, then drill the upper tank develop some clear acrylic spouts to put in the hole, and rig a canister filter to suck water out of the lower one, and pump it to the upper.
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