Tail sitter with vectoring thrust and 2 separate batteries

Thinking of converting my Fx-61 to a tailsitter.
Came up with an idea of using 2 small multirotor motors with vectoring thrust in front, which will have a small 5-6s dedicated battery, just to get the model airborn and land with the purpose of saving the main low-amp li-ion pack.
The third motor is sitting at the back, as usual on Fx-61 powered by the main battery pack.
My concern is, will it be possible to program the controller, that when it transitions to the horizontal flight, the front to motors are shut off and the rear one kick in?
Another idea, is to apply a limited amount of power to the rear motor after lifting off (in multicopter mode), to help the front motors. Hard to believe this part can be programmed, but perhaps, can be done with a switch?!