Raul, I don't know what your weight limitation is for your winter treks but why don't you try using both of your Yetis in temps below 20? You'd be adding 11 ... 12 ounces with the 3 season? Less than a pound for a good night's sleep. Might be worth experimenting with since you already have both.
And there's always the hot water bottle in a sock trick.
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