You should cut each 2X12 individually and then stack them. You would of course cut them on their flat side, so any skill saw should be able to make each cut.2) still not visualizing how to do it with one cut. Maybe with two?
You can more accurately follow a fence than you can follow a line. So clamp down (or even screw down) a fence to act as a guide for the edge of the skill saw (offset from the desired cut by the distance from the blade to the edge of the base plate). Then you run one edge of the skill saw's base plate up against the fence as you cut.
Most skill saws have a motor sticking out to one side. So if you're using clamps to hold the fence the motor will run up against the clamps. So you cut in the direction where the motor is on the open side (and not sticking out into the clamps).