See this video
The audio is the orchestral version of Shosty 7, but the video shows a two piano arrangement, which is what the question is about. If you look at 1:06:17 through 1:07:46, you will see that the first piano gets told to play pp no less than four times, and the second piano four times as well. It is even sempre pp every now and then, which to me seems only to further stress that the volume should stay the same. There are also no crescendos or diminuendos in that segment.
I was working on arranging that bit for solo piano for my own purposes. Should I take over every repeated instance of pp, even when they do not seem to change the dynamics?