Wiring up dual batteries

[B_derK20]

I've recently parted 2 trucks. Each one had a dual battery setup. The diesel was simple with the long positive cable from battery to battery. The gas motor was more complex. The batteries were not connected to each other. The auxiliary battery had a wire running to a relay and then a wire to the distribution post on the firewall my question is assuming I get 2 fresh batteries so they don't draw each down, can the two systems be combined so I get the benefit of the extra battery for cranking as well as a way to power other accessories or are the 2 methods incompatible or otherwise a bad idea together?

Thanks!