Racing battery question

We ran 2 batteries in the back without an alternator. We had isolators on each so we could run off both or just one or the other. It's handy like that if you start going rounds and have no time to charge. All you do is switch over to the fully charged battery on board. At our track they often run semi finals and finals very close together to wrap up the day so everyone can go home and we've been almost stuck without time to charge.