Our Alpkits Skyehigh 800s have opposing zips so that they can be joined, i’d imagine that any otherwise identical bags with opposing zips could do the same.
BUT – it’s generally much warmer if each of you are in a single mummy bag. As soon as you move apart in a double, even slightly, you let out so much heat.
Individually, our bags are comfortable down between -5 and -10, depending on how tired we are/when we last ate etc….but zipped together we start to feel the cold at anything less than +3 roughly. Waking up in the middle of the night with your partner a couple of feet away and therefore nothing covering half your body isn’t ideal when it’s cold 🙂
IMO, if you intend to zip two bags together, but ones that are rated at least a full season colder than what you need, and be prepared to separate them in the middle of the night if you’re getting a chill.
As a nice surprise for your other half, buy two good 3 season bags with opposing zips and get her an Alpkit Filo jacket (or similar). Draping a down jacket over her bag will work wonders if she feels the cold. It’s probably cheaper doing that combination than buying a lightweight but super warm sleeping bag. Also, don’t forget good mats, you lose an incredible amount of heat to the ground if not properly insulated.