Any two bags of at-all similar length with the same type of zipper (e.g. YKK#10, etc) can be zipped together. However, unless unless they are a left and right bag, someone ends up with the "head rest" portion hanging in their face.
If you are joining buying two new bags, typically women sleep cooler so if her bag is rated 10-15F cooler, you'll both throw back the covers (or bundle up) at about the same time. (also, you'll have more variety to fine tune on solo trips).
Also, as a double bag, it will never seal around your neck as well as a solo bag does. So bring a little warmer bator wear a little more around your neck and shoulders (I've got a fleece dickie that works nicely).
A few tricks on sleeping pads: Thermarest sold a coupling kit which was just a loop of nylon twice a Thermarest in diameter. Half a twist in the middle and you slip one pad down each loop, at shoulders and knees. A very easy DIY. There are also pads that come with velcro on each edge – hook on one, pile on the other. That works even better, but rather than place it on the flat edge, I'd place one bit inward from the edge so they overlap with constant single-thickness coverage and no gap. It doesn't have to be continuous velcro. A 2" x 3/4" patch every foot or so is plenty. Some velcro comes with an adhesive backing. If not, a bit of fabric cement will do. Any solution is better than nothing so pads won't slip out and away from both of you.