Lot depends on how well they go together... if one restricts the other from full loft then you won't get a lot of gain...
First estimate take the send bag reating subtract it from 70, devide that number by two and then subtract it from the first bag rate for a combined value ex
Two 40 degre bags... 70-40 =30 devided by 2 =15 subtracted from other bag 40-15=25 for a first cut estimate of the combined bags... it may actually be better but probably not worse, so this works as a good conservative estimator, IMHO.
Ounces to Grams.
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