I think your best option is to order a package of the warm and plush for this particular quilt. You will make more quilts that you can use the warm and natural with. I would definitely not double up the W&N, not only for the reasons stated above but also for purposes of washing. A quilt with two layers of W&N which is a very heavy batting (weight wise) to begin with will be so heavy soaking wet, it could easily damage a washing machine in the spin cycle. Also if you are not one to quilt closely, the extra weight of the 2nd batting, when wet, could result in popped stitches, both seams and quilting.
If you truly want to double up, use only one layer of the W&N and layer a much lighter weight loftier batting on top of it, like wool or poly. Wool will definitely add a layer of insulating warmth and loft without adding much weight.
As someone who has done a lot of double batting I do have some experience. I do quilt on a LA, however. I can't imagine how difficult it would be to manipulate any double batted quilt on a DSM, let alone one that would be as heavy as double W&N.
Another option is to simply make your dad a second quilt so he can layer the quilts up.