I would say that it's fine for Rainier. Layering two down pieces together is difficult, because down gets less warm the more it is compressed. Basically, you want your down layer on the outside, and other things below. I would suggest that if you're wanting two jackets to pair, buy the Ama Dablam, which is a fantastic jacket in it's own right, and pair it with the Marmot Alpinist Hybrid, which is one of the coolest jackets on the market, and will pair very well. That said, for Denali, you may want something more than these. Windproofness will be essential there, and it's a lot colder than Rainier. I would look at something like the Greenland baffled jacket, if you need a belay/rest jacket, or maybe the Marmot Trient overtop of the Alpinist Hybrid, with several other layers underneath. The best idea is to dress in exactly enough to keep you warm when you're moving, and then put on a puffy jacket when you stop. The Trient is a hardshell lined with 60 grams of Thermal R, so combined with the Alpinist Hybrid, they should keep you plenty warm when you're moving. When you stop is when you'd want to put on a jacket like the Ama Dablam overtop. Hope that's helpful.