I use two down bags quite a bit. Works fine for me, just mix and match according to the expected temperatures. I use Western Mountaineering’s Sleeping Bag Comparison Chart to estimate how much down is needed. (and also just for window shopping)
Examining the chart infers that an estimate for the amount of down required at an expected temperature is given by: Ounces = (55-T)/2 where T is the expected temp in degrees Fahrenheit. That is, for 30f you would need (55-30)/2 = 12.5 oz down. For -40 f that would be:(55-(-40))/2 = 95/2 = 47.5 oz.
Basically, an ounce of down is worth 2 degrees Fahrenheit. So mixing a 40 f synthetic bag with a 12 oz down bag would give you 40-12(2)= 16 f.
Inherent in this estimate is the assumption that the down is working fairly efficiently and is not overly compressed or overly loose fitting.
Like others have suggested, I do a lot of backyard testing when a cold spell comes to town. Try it, the neighbors probably suspect you’re a nut anyway.
edited because I am eventually gonna learn to make a link.
