You treat 2 6v batteries wired in series the same as a 12v battery. A 12v battery has 6 cells wired in series a 6v battery has 3 cells, so two 6v batteries wired in series have 6 cells just like a 12v.
You may ask why PC uses 6 v batteries. The batteries they use are more expensive and designed for many charge-discharge cycles. They will last longer and work better than the old 12v batteries they used a few years ago.
The truth about charging batteries is the converter charger does not put out enough voltage to fully charge your batteries. That's why a solar panel with a charge controller will get the batteries up to 100% charge.