A question that was not asked,.....
If they were tested individually, which they should have been,...were both batteries tested as bad ? Its very unlikely both would go bad at the same time. Yes, I know all about one bad batt will take its toll on the other.
If one was much better than the other, then its likely only one of them failed, and would be considered "defective", which happens to even the best deep cycle battery now and then.
I wonder if the batteries were charged before any kind of test was done, which I believe is the only way to get a true test,....even tho some "smart testers" claim to be able to test 1/2 dead batteries.