Insufficient info. How exactly is it wired now? Do you want to charge all 8 batteries from either alternator? Do you want to do that automatically or by manual switching?
Keep in mind that charging batteries in parallel, especially if they are of different design and age, requires some charge sensing on each to make sure they get equally charged. There are commercial charge controllers that do what you want. Perhaps easier in the long run to fork out money for one of these.
Klaus