By both sides, do you mean two layers of planes? If so, you're going to have to cut twice on both sides. Press K to toggle the knife tool. It's easiest to use in either the edge or vertex mode, but I reccomend the vertex mode. Start by clicking where you want your cut to begin, either on an existing vertex (the little square should go red or green) or on an edge. Then click your ending point on another edge or vertex. If your cut is finished, press enter. This new edge will be selected along with whatever you had selected before you started cutting (if you had anything selected). I hope this helps!
There's somethign that is even easier to use. Select Object -> Hit Tab (to enable Edit Mode) -> Hit K (to enable knife tool) -> Hit Z (to enable cut through option) Video - Click to Play - Direct Link
