The media keep reporting that the two pilots did not ask to fly together. That of course predicates that it was 'essential' that it was this flight that was chosen.
It might equally have been that the two pilots waited until chance put them on the same flight.
This is not to say that the pilots were the culprits but that not asking to fly together is a red herring.
I think a better question would be whether either of the pilots asked to be on that particular flight.