It's definitely not as spiffy as a pre-made groundsheet, but you can easily get a matching zipper from Seattle Fabrics and sew your own. I suspect it'd be better for climbing than whatever they'd make, as the real target market for their two-person systems is romantic, not climbing, partners.
You'll want to (A) get the lightest (i.e. least comfortable) possible fabric and (B) sew the sheet as a triangle or isosceles trapezoid narrowing toward the foot. I believe Rolando Garibotti's Patagonia book has a whole schpiel about setting this up.
Frankly though, I've used the quilt with two people by just unzipping the top half of the bag and tucking the edge under my shoulder. It's not totally windproof, but good enough if you're in a bivy tent or in a sheltered spot.