Mike Wills used the 2 motor cycle battery trick for the RV4.

The situation is you use both batteries for starting
and then, after a short while for charging purposes, you switch
one off for flying.

The other is kept in reserve in case of alternator or battery
failures. This probably puts a glitch on the main power bus when the
back up battery is switch in or out.

I will add a choke and large capacitor on the main bus to smooth
the glitch.

Paul