Ok are you saying when you build your foam wings you want to use wood joiners like the ARF built up wings use ? If your answer is yes. then you would have to cut slots in both halves the length of the joiner . Just make sure the wood grain runs length wise not up and down or it will break in the center. You could use l/8 light ply about 10 to 12 inch long and what I would do is cut the slots 6 inch each half before you put on the veneer skins. Do a dry fit slide the joiner in place and put the wings together making sure its a snug fit. After you are satisfied with the fit sheet the wings with the veneer making sure no glue get into the slots. After the skins are dry slide in the joiner do a dry fit again to make sure the 2 wing halves fit nice together . take apart and I would use Gorilla glue in the slots to glue the joiner. The gorilla glue expands and would fill any voids in the foam. The amount you use depends on how snug the joiner fit. If its snug just a thin coat of glue on the joiner , spray a little water in the slots. Do one side and slide the joiner in place to the half way point. don`t forget to mark this on the joiner. Then put epoxy in the middle surface of the wings and slide the other wing half on. Wipe away all glue oozing out and tape wings together with masking tape to keep it straight. I use wide tape for this . Along the joint top and bottom and across the joint. Hope this helps.
Note you do the same for wings with dihedral . You just have the cut the angle in the joiner and also the foam wings.