Why would I need two sleeping bags on Everest or Aconcagua?
Because on these two mountains you live in two places at once. There is a base camp you keep coming back to, and a camp higher up where you sleep on your acclimatization rotations. Carrying one sleeping bag up and down between them on every rotation is hard work, so many climbers keep one bag in each place. See Where your gear lives between camps.
The full answer, kept up to date: Why two sleeping bags on Everest or Aconcagua?