Why not use an inverter to operate a 110v. queen ?
or as suggested, use two 48's with individual heat control.
3rd option is to use two 36x60 12 v mattress pads. avl from tractor trailer supply companies .
Our solution: 110v queen mattress pad off an inverter to warm things up . Once that's done, our King size double sleeping bag takes care of business better than anything we have ever used. Most of the time its too hot and it uses no power. We bought two king size top sheets and had them sewn together as a washable liner.
We found the king size to offer lots of wiggle room on top of a queen mattress.
http://www.rei.com/product/828229/the-north-face-dolomite-3s-sleeping-bag-double