For a sleeping bag to perform the best, it needs space to loft to its maximum. By stuffing one bag inside of another, you might end up compressing the insulation of the inner bag to a point where it won’t do you much good.  If buying a new bag is not an option, you might look into renting one, or borrowing one from a friend. When we trekked to EBC few years ago, our trekking company arranged an inexpensive rental for us in Kathmandu.  Sleeping bag liners are also a great way to add warmth. They come in a variety of temperature ratings with the cold weather fleece ones adding 25-30 degrees. Unlike sleeping bags they do not need to loft so a combination of a bag with a fleece liner might give you better results than using two sleeping bags. Hope this helps. Happy trails! ... View more