Cut two slots in the middle of the top of one wiper blade, the same distance apart as the two sides of the blade. I used a Dremel with a cutting wheel 'cause I didn't have any hacksaw blades, but a hacksaw should do the trick. Don't cut all the way through. If there are holes in the blades already, it's fine to cut into them, or not.
Slot the other blade in to the slots at right angles. Depending on the accuracy of your cuts, the tension may or not hold them together. If so, great, if not, never mind, the wire hook will hold them together later.