@khuey Thanks for reaching out!
We're glad you love your sleeping bag! The trick to connecting two sleeping bags together is making sure the zippers are on the opposite sides of the sleeping bag and that they are compatible zippers. Almost all of our REI sleeping bags are made with compatible zippers, so you'll want to find a sleeping bag for your partner with a left zipper. The most similar sleeping bag to the one you have that would zip together with it would be the REI Co-op Igneo 25 Sleeping bag.
Hopefully this helps!