(ET) Battery/charging question (not elec trak)

["Chapin, Tim" <tchapin umich edu>]

I am electrifying my pontoon boat this spring with a big trolling motor
(24v).  I am also leaving the gas motor for when batteries get weak or the
kids need to go to the bathroom (speed run to the house).  I was thinking
about keeping the weight down by only having 2 batteries and taping of the
first battery for the lights and electric start for the gas motor.  The gas
motor takes very little to start so as long as I educate people not to run
the trolling motor until it dies, that shouldn't be a problem.  Battery
balancing will not be a problem because I am going to use a bank charger 
and
charge the batteries separately.  My question is, will I hurt the gas 
motors
charging system (ie. Would the motor charging system be isolated) by just
taping in to the first battery or do I need to add switches to isolate the
gas to 12 volts and isolate the trolling motor to 24 volts?