The Knuckleheads are tethered together by a unbending chain, so controlling them is a bit like steering a giant dumb-bell. Hold [right] to rotate clockwise and [left] to go counter-clockwise. One of the heads will grip the earth, allowing you to pivot the other through the air; they travel end over end, taking big, clubbing steps across the ground and plowing along sticky walls and ceilings. You can extend the chain linking them together by pressing [up] or retract it with [down], in order to bridge gaps or step between spikes. The [WASD] keys serve the same purpose as the arrow keys.
Combat involves bashing enemies with either the green Knucklehead or the purple one. Conveniently, almost every character in the game comes outfitted in purple and green as well, so you know which head they need to be pummeled with.