Chris Marshall: My marking knife is only sharpened on one face of the blade -- but with two beveled edges on that face that meet in the middle. The back face, like a plane iron, is flat. However, master woodworker Ian Kirby uses an ordinary pocketknife from time to time to score knife lines, and the blade is, of course, sharpened on both faces. It can work both ways, but I like to have the flat back face as an easy registration surface against my rule. You can use oil- or water stones to sharpen the bevel or bevels of a marking knife, just as you would a chisel or plane iron. Keep the back face flat, hone it if needed, then touch up the beveled edge.

Tim Inman: It is a personal choice, but I sharpen mine with the bevel on one side only. That way I don't have to factor the distance between the cutting edge and the edge that follows against a ruler, etc.