There's only a place for one house battery in my B+. I added 136ah capacity with these gel batteries from a UPS (uninterruptible power supply). The are meant for indoor use and are normally housed with an inverter and charger built in.
I can choose from (1) original house battery, (2) new 136ah battery pack, (3) both combined, or (4) Off, with the battery selector switch.
Originally, I used this idea in my Roadtrek. More info here: