I don't necessarily agree with putting the synthetic bag on top. Most likely which ever bag is on top will stay dry. In temps as cold as -20C your body temp shouldn't be able to push vabor through 3 fabric layers before it condenses. Also the new down bag is lighter so most likely won't compress the heavier sythetic bag as much allowing you to capitalize on more loft. The most important thing is loft so take the two bags and open them all the way up and layer them together and see which layering gives you the most cm of loft. My guess from when I did it with a 3 season down bag and a summer synthetic bag, is that your down bag will be on top because it is lighter. When you go to test it you want to fully zip up the inside bag and use the outside bag as a quilt overtop.
The most important thing however is to test it in your "back yard". I don't really think you can do any amount of math to come up with how cold the 2 bag system can take you. Too many variables, your body, loft, compression, vapor... Just test it on single overnights until you hit either as cold as you want to go or as cold as the double bag will take you. Eat a big meal too!
Also you can try a vbl (emergency foil bivy) but they are hit or miss. I really don't like them at all. Don't underestimate the power of the sun it may be -10C out side but in the sun with a little breeze a bag will dry out enough to retain its loft for several days of a trip.