Myth
It is quite undesirable to put a sleeping bag in the inside of another sleeping bag. You can't really achieve your purpose.
Reason: simply stated, the thermal rent rate * thickness = thermal resistance (commonly referred to as R value). In the absence of structural problems (wind resistance, different thickness of the human body), the same filler, the thicker, the better.
When the sleeping bag is applied, the inside of the sleeping bag is compressed by its volume, and the thickness is reduced. Even with the same filler, its warmth will not reach the original state of warmth. (in fact, the rate of heat rent will also decrease, and the feather will stick together)
In the most common sense, it is the feather that can't keep warm.
In addition, if you are thin, the inner sleeve is not bad, but it is recommended to be within 200 grams. Otherwise the inner space will be compressed. It must be noted that if the sleeping bag is tight or loose (like SD), don't think about it. In addition, as a basic sleeping bag, if it is a super light product, the inner sleeve is also very undesirable.
The following methods are used as an emergency. Consider investing not as a waste, not as a first consideration.
Camp like you go hiking, hiking routes is the basic temperature of 0 degrees, in order to guard against possible suddenly cool, you can prepare a - 3 ~ 5 on the basis of sleeping bags, match again a + 3 ~ + 7 youth hostel sleeping bag. That's when you're more convenient. The total weight is slightly heavier than a minus 10, but you are more flexible. A +7 sleeping bag can be used as a supplementary sleeping bag or as a supplementary down jacket. It can also be a quilt for your hostel, and you can also reduce your investment as a three-season sleeping bag.