I am looking for a wall mounted speed controller for two Harbour Breeze fans being fed from a single switch. Any suggestions? I currently have a slide speed control installed but it produces a hum.

Technically speaking, controls are only supposed to operate one fan at a time, however you can usually install two fans to one control as long as it falls within the amperage ratings. Any 3 or 4 speed control (NOT variable speed) should be fine.

Ceiling fan remote controls - help and FAQ- Ceiling Fans N More