As I understand it, the assignment had nothing to do with the request from the pilots.
I don't know how those such things work in the airline industry, but it would therefore have to be agreed upon before the two:
"If we ever have the chance to fly together in the future, we're going to do such and such..."
Otherwise, it happens right then and there.
P1: "Say, you know what sounds good right now?"
P2: "Lay it on me."
P1 "Taking this thing to...."
