Yes, just wire the 2nd house battery in parallel (+ to +/- to -) with the existing house battery. They will become one large battery. 2 things to be aware: 1) Combining and older/used battery with a new battery will bring down the new one as the they will equalize. This may reduce the overall life span of the new battery being added in. The age and condition of the existing battery will determine by how much. 2) How deeply this 2-battery house bank is depleted on a given trip, will have some impact on how effective the existing ACR continues to be.