before you can do aproper analysis you must ensure both baterries pas a proper load test that is half the cca rating for 15 secs the batteries must be issolated and tested individually if they both pass then you can test alts on the truck with same volt amp tester because you have a dual system you may have a simple connection problem at one of the alt besu all with one pire to check the conectors at each alternator for signs of burning any good repair facility can check both batts alt volts and amp output [max] and running starter draw volt drop with one vat tester if you want to do it yourself you must take batts to someone with a load tester capable of 400 t0 500 amp draw for 15 secs then check outputs with dvom hope this helps good luck dennis