Opinions vary on whether DA can withstand heavy and wind driven rain (and no discussion of snow that I could find.)
I have observed that at horizontal laps, the top layer will look soaked while the bottom layer is dry.
Also, after a rain, DA dries like cloth: the water wicks downward so the upper portion will be dry while the bottom will be soaked - the bottom edge the last to dry.
It seems to me that two layers would address all these concerns - including UV where the top layer would protect the bottom layer and extend its live beyond the 90 or 180 (I don’t understand the two specs for DA.)
I have some splice DA areas that will be exposed over winter (Pittsburgh, PA) and I’m thinking just another layer instead of a tarp.
Regarding PERM: One might argue that 2 layers cuts the perm spec in half and is undesirable. Since asphalt shingles’ perm of about 1 makes them almost a vapor barrier, a double layer will not have much effect diffusion of water vapor through the shingles - half of nothing is still nothing. Evaporation of trapped water into the structure would get the same argument. By the time you make it through the sheathing, even a double DA layer would still be a high perm.
NOTE: I may try one of the SIGA underlayments for the 2nd half next spring.