You can join them with any foam safe glue. Yes, it is a weak joint. You MUST fiberglass the center of the wing with foam/balsa construction because all of the strength and stresses are in the balsa skin. I would use about a 6" wide strip of 4oz fiberglass at a minimum. You can also laminate several layers of lighter glass, like 2oz. The pylon racers usually use about 3 layers of 2oz cloth, starting with a 6" wide strip, then a 4" wide strip, then a 2" wide strip. You can use laminating epoxy if you have it, it is the best choice but takes 24hrs. to set. You can also thin some 30 minute epoxy down and use that, this is usually what I do. Whatever you use, make sure you fully wet out the cloth, then soak up the excess resin with toilet paper or paper towel to keep the weight down to a minimum. You don't need to use one strip of cloth to do top and bottom surfaces, either. One over the top and one over the bottom is easier to keep smooth around the LE area. The best way to cut the cloth is with the fibers at a 45 degree bias, but straight is really OK for anything but the most severe usage.

Mark