I have 3 tanks stacked and I am using the light from the middle tank to shine into the top tank. It is fine, the fish adjust and no they don't swim sideways or upside down, at least not after a day...My Oscar didn't take to the situation as well as others.
I currently have a heat lamp over the top tank that is much brighter than the botom glow, but this is due to the weather and the locations of the tanks, this is a cold water tank, so the heat lamp is only to see activity... 63* last check.