On one of the boards I visit, someone had 2 heaters running horazontally across the back of his tank. They were about 3/4 of the way down the back glass. He did this so they would be pretty much hidden behind his decor/plants when viewing the tank. Worked very well for him.
Much easier to hide a cord than a whole heater...or two.
Yea, thats how I have mine, 2 200 watt tronic heaters horizontally in the back of my tank, about 2 inches or so from the sand. You would never know they were there except for the cord.