Using Two Sleeping Bags / Quilts for Winter Camping

Can I combine my sleeping bag and my quilt to get a good winter sleeping bag? I have a synthetic, big, heavy winter sleeping bag, an ultralight 3-season down quilt and an old down summer sleeping bag. How warm is my quilt and my summer sleeping bag together? Here is some math I've found on the internet:

For Celsius
Comfort temperature from one bag (10) + comfort temperature from the other bag (0) - most comfortable temperature (25) +7 = the new temperature raiting
10+0-25+7=-8°C

For Fahrenheit
Comfort temperature of the warmer sleeping bag – (70°F- comfort temperature of the colder bag)/2 = new temperature raiting
With my quilt and sleeping bag:
32-(70-50)/2=
32-20/2=
32-10=22°F

Don't trust the formula completely! It will be only approximately correct. Even with the two formulas above I get results with a difference of about 2.5°C or 4.4°F.

One thing I know for sure when I've used the sleeping bag combination in -5°C/23°F. I had a good and warm sleep, which I wouldn't get with only the quilt. I don't know where these formulas came from. I've found them on a few different websites.

Sleeping bags, quilt and other equipment in this video:
Thermarest Vesper Quilt -6C/20F
Mountain Hardwear Lamina -18
Some sort of summer sleeping bag
Therm-a-Rest Neoair Xtherm