Hi Skip
Some yrs ago my12 month old Lowrance sounder /GPS lost pixcels and Lowrance advised me they would honour warranty and replace the unit but unless I powered my electronics from an isolated (from the engine) house battery there would be no more warranty.
I had a house battery setup installed with an ABC that isolates the motor battery from the house when starting or when battery volts were too low. No more issues and no more need for warranty.
Maybe electronics are now smarter and are no longer affected by spikes etc from starting motors but thats a question I cannot answer.
Re staring a motor when the battery is flat , my set up allows both motor batteries to be linked if one goes flat.
I.E. you start the flat battery motor with the other motor battery then isolate the good battery from the flat one and start the second motor. The first motor with obviously run even if its battery is flat unless you have one of the new fangled motors that need their battery to be in tip top shape or you reach for the oars or phone for a tow.
Cheers
Chimo