We are going to use B6 for the DCPD BS (BS2), and B1 for the QPD BS (BS3). Their role can not be swapped.
B6 has the best loss among the available ones, while the perpendicularity is not so critical due to the short arm.
B1 has the OK perpendicularity, while the loss is also moderately good.
The attachment is the excerpt from T1500060 with some highlighting.