Ok so we TAXI out.
You say PF needs to start the engine during taxi.
So he hands over control to PM.
PM now becomes PF.
The new PF now has to hand control back over to the new PM.
And so on and so on..
I can see that going back and forth for a while..
Why on earth would you want the guy who is taxiing and supposed to be looking outside, starting an engine?