Your code contains a call to SPI.beginTransaction(), a for loop with calls to SPI.transfer() with the SS pin LOW, and a call to SPI.endTransaction(). That part, you say, works.
Then, your code contains a for loop with calls to PI.transfer() with the SS1 pin LOW. That part, you say, doesn't work.
Gee, I wonder if those beginTransaction() and endTransaction() calls just might be important.