Jonas,
After I glued the 2 halves together, mine was in one piece also. The plans show where to cut off the front piece, like the picture above shows, so you end up with a smaller front piece and a larger back piece. You then need to glue 2 pieces of 1/8" plywood to the inside of the smaller front piece, 1 on each side. The plywood strips are 1/2 x 2 1/4 x 1/8. The plywood overlaps into the back piece when the 2 parts are put back together. The #2 screws are used to attach them together, 2 screws on each side. I've attached pictures to show the mounting bracket under the wing where the legs get attached to. Take another look at the plans and at my pictures. If you're still not sure how to proceed, I'll try and explain further and take some more pictures.
Good luck, this is one of the trickier parts of the kit but I know you can figure it out.
Mike