While someone MAY have wired it that way (2 6VDC batteries in series for just the inverter and the other two for all other house duties,that is not how I would do it, though certainly not difficult to do.
But, due to Peukert's law, you will get more total amp-hrs of use from one larger battery bank than two smaller ones. Said another way, the higher the amp load vs total amp-hrs stored in a battery, the less total amp-hrs the batteries can provide.
To wore the 4 6VDC batteries in to ONE bank, you will wire each pair in series (to make 12 VDC) and then each of those in parallel to make a larger 12 VDC battery.
Plenty of on-line diagrams of this.