are the both conected together and were they both new at the same time
I seem to remember that there was a spate of Yellow Top problems a couple of years back. It may be worth checking whether yours came from the same batch and, if so, seeing if you can get a replacement from another batch.Just a thought.
If the two are in parallel and are not dis-connected when not in use/Charging. I am led to believe that the one with more amperage left in it, will trickle charge the other and so on back and forth untill both batts are duff.Could this be the problem?Easiest way to sort is a manual switch to split them when the motor is not running.