On boats with symetrical ends like the QuickLap canoes you can assemble the matching plank on top of the one that you just finished. Lay one of the matching plank pieces on top of this assembled plank with it's scarfed side up and it's scarf joint area at the opposite end from the first plank. Line up the ends and edges with the plank below and put waxed paper under the area where this scarf is. Place the other plank half on top of the assembled plank as well, so that it's scarf area is also on the waxed paper.
Using the same procedure that you used with the first set of planks, wetout the scarf area with some of your un-thickened epoxy mixture, and allow it to soak in. When the scarf area won't absorb any more epoxy, apply some of the thickened gluing mixture to this pair of scarfs.