layering sleeping bags?

My brother is off for Denali in a few weeks. He purchased the bison bag with extra fill for $800 I believe. But he loves mountaineering and climbs all the time so it was justified to him. Whats funny is he recognized that he will never need the huge down jacket that he wants to bring for this trip ever again, unless everest or k2 come into play. He has decided to rent the jacket from the company he is climbing with. Its all a personal choice but if you only plan on climbing denali now and possibly everest later, i would rent. Besides if you decide to do everest you will have probably won the lottery or invested your money well so you can splurge then

I find it hard to believe that your Highlite would fit inside the -20 degree REI bag without squishing the loft in one or both. Since loft is basically proportional to warmth, you wouldn't see the full temperature benefit if it's squished.

Some manufacturers make overbags that are expressly designed for adding warmth to another bag. A custom built quilt could serve the same purpose. HOWEVER, I personally wouldn't feel good about a double bag option on Denali. Maybe some with more experience would. I have seen -40 degree bags in gear lists for Denali, so some people apparently think that's the way to go.

Henry, I have tried layering my two Montbell Super Stretch down bags, and due to their unique circumfrential baffles, they fit inside each other with minimal compression. I believe combining the fill weights to estimate the temperature rating of the resulting two bag combo would be accurate; for example a #3 inside a #1 might get you to a low temp of -40; while still having a three and a light four season bag in your gear closet.
Id hate to be the one to test out this theory down at -40.. but I can vouch that even a #7 can be used at 25F with some layering.

Using a two bag combo for extreme cold is a well accepted tactic for climbers. I know a few who use that system for extreme cold, but I don't use that system personally.

Mind you, dediacted alpinists have been climbing himalayan peaks with 0F bags for decades, so keep an open mind about what is necessary for succes if you're willing to compromise on confort.