you could try just one powerhead directed against the glass, creating a "gyre" or vortex type flow. I use it in my rectangle 55g, but it works better in hex or cylinders.

Never had any cyano problems or dead spots

I currently have the two powerheads directed at the glass, but both come out of one spout so i'm having the feeling that the other 'side' of the tank is getting enough flow. Keep in mind it's a center overflow. 28" diameter, 28" high.