Trying on what you intend to buy is great advice. Also understanding what is meant to be layered with what, does too. Can you layer the DO under the DWJ? Yes. It’s a little bulky in the upper, though. The DO is designed primarily an outer layer jacket. The Fahrenheit Jacket (I find the Fahrenheit and DO to be similar in warmth, just cut differently and fill different roles) layers nicely under the DWJ for extreme cold and the Gradient jackets will get you through a lot of the season with the right base layer.
One of the easiest ways to understand Sitka’s waterfowl layering vs outerwear is if it has upper hand warming pockets on the chest, it was designed to be an outer layer. There are outliers, the Dakota hoody is one, but for the most part in the waterfowl line, upper hand warming pockets equal outer layer, normal lower location pockets equal layering piece. Of course you can do whatever works best for you, but that’s the general rule of thumb I tell people.