Eins is right.
Unzip the sleeping bag all the way. At the very bottom, you will notice that the zipper separates into two pieces, like a jacket. One side of the zipper will have the slider(s).
(Many bags, including I think both of yours, have double-slider zippers, so you can ventilate from the bottom. This slightly complicates things, but it still works the same.)
Now place the bags side by side. If this is going to work at all, you'll find that the slider on one bag is on the bottom side of the zipper, and the slider of the other bag is on the top. Hook up the two bottom zippers and zip them all the way up. This will give you a single large bag with two hoods on the bottom. Now connect the top zipper sections, get in the bag with your friend, and zip it up.
If the sliders are on the wrong sides, it won't work. If you don't have separating zippers, it won't work. If the zippers are very slightly different, it won't work. If it seems to work but feels very stiff when zipping up, you might want to skip it.
Finally, note that this set up is colder, in general, than separate bags.