Oh crap... That's how we have our QT tanks now. There's a rubber mat dividing the two but it's a typical metal tank stand for 2 twenty gallon tanks. We divided them in half with glass & silicone for more individual tank space. I guess that was not such a good idea after all.
I've had my QT tanks this way for a while, but hadn't actually been running the upper one. Probably cross contamination is rare, but clearly can happen.