..... why would you. Placing the Bus Tie Switch ON will load up the battery with the services on the Main 2 BB - not so smart if you are short of battery volts.
It is possible to start an engine with less than the required 23v but this is not recommended by the engine manufacturer because the resulting engine acceleration is too slow. This results in a low cooling air mass flow through the turbine area internals. This can lead to extreme heat in areas where the subsequent excessive metal expansion causes blade-rubbing on the inside of the turbine case. At the next internal inspection, the engine would be pulled for an overhaul.
A 'one-off' such start in extremis would probably be OK but those that have done it habitually have paid the price.
G.
