I am not Ryan, but here is my interpretation of what Ryan said in the article.
Ryan: As conditions get stormier and colder, I opt for a 2-shell system (wind shirt + rain jacket). This adds a touch of warmth in the air layer trapped between the two shells, and much better storm protection in very heavy rain and high winds.
The wind layer has several possible functions when used underneath a rain layer:
1) It is an extra layer of insulation. It is not better than Alpha or some other insulator, but if you are carrying a wind layer then it is insulation that you have with you.
2) In prolonged rain people usually get wet. They often blame “leaks”, although I suspect that a more common culprit is condensation. Another hypothetical way for water to ingress is by overpowering the hydrostatic head of the membrane. This has been discussed on BPL in the past. The short version is that it depends on the wind and the size of the raindrops, which vary, as well as the specific membrane. There are many anecdotal “suspicions” of this kind of water ingress, especially with electrospun membranes. I have not been able to replicate it underneath a shower, but that does not mean that it can never happen.
Inside of a waterproof layer, you will get condensation for sure. Inside of a WPB, once the DWR wets out, then you will get condensation as well — possibly more than with a straight waterproof because the wet-out layer is cold.
In a perfect combination of weather and low output, you might stay dry inside of Outdry/ShakeDry that cannot wet out, but only if the outside vapor pressure is lower than inside, which requires some luck.
At minimum, it is nearly impossible to avoid avoid some condensation while active.
One way to deal with water under the rain layer is to wear insulation that works when damp, such as fleece (especially Alpha Direct or Octa/Airmesh which retain little moisture). You will still be damp, but you will be warm, and your body heat will eventually move the moisture away from you. Eventually, after the rain stops, your body heat will “cook” the system dry.
In warmer weather I like to wear just Brynje under a WPB, at least when I am not around other people.
Another way to deal with condensation or water ingress is to wear a DWR layer inside of your rain layer. With any luck, condensation that forms inside the rain shell will roll off of the wind shell. The wind shell may also provide some defense against rain driven through the rain layer (if such a thing really happens). So there is a possibility that a wind shell inside of a rain shell MIGHT BE dryer.
In any case, a wind shell won’t be any worse than wearing fleece under your rain shell.
3) One more advantage is that, when the rain stops, you can take off your rain shell and still be protected from wind. This is valuable if you are wearing only Brynje or AD/Octa under your shell(s).
All of these effects are subtle, and may occur only in specific conditions, which is probably why you don’t see the advice more often. Some people switch back and forth between wind and rain shells. Others wear only a rain shell and skip the wind shell completely. Still others wear two layers of rain gear in heavy conditions: WPB (for comfort and versatility) with a poncho (maybe an emergency poncho) over it for the cold/driven rain that Ryan describes.
TLDR: Wearing a two-shell system is an interesting alternative for cold/driven rain.
Choose what you like for the conditions you face, and be aware that other combinations may be possible.
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