batts dont typically just die. They start to die slowly. With two 6's one might be going bad, the other still is good. The two add together to still give you enough volts to do everything. 11.5 is low, but 5.75 plus 6 is 11.75 which is still pretty good. With one 12 volt batt only able to get to 11.5 and the other 12 then the total voltage available is still 11.5. The bad batt brings down the good one. when parallel, but not when in series. This is the big reason to go with two 6's. You can mis-match two 6's in series, but you really should only exactly match when in parallel (two 12s).
Nonsense? No. Good advice, but go do an easy forum search as there are many post on this same topic and easy easy to find with a few keystrokes....
* edit. General definitions when talking about wet batts is the 6 volt "golf cart" batts are a true deep cycle batt with thicker plates.
The 12 volt batts typically talked about are the "Marine" batteries that are not a true deep cycle batt, they do have thicker plates than a car starting batt, but thinner than the GC. Marine batts are meant to be a compromise for boaters (marine) to start the boat motor, then do some trolling on the little electric motor, then start the boat motor. A car battery would die trolling, and a GC batt might not have enough surge of power to start a big boat motor if not near full charge.
Unless specified these are the 6 and 12 volt batts being talked about. If someone has special 12 volt true deep cycle battery then they should state this so we do not assume it is a Marine batt.