Or, better, use a Xantrex EchoCharge (15A maximum current flow) or a Balmar DuoCharge (30A max). These work by sensing a charging current on the house batteries, then diverting a portion of it -- up to 15A or 30A, respectively -- to the other battery bank to be charged.
The banks are kept separate, no significant voltage drop, no in-rush current when the batteries are "combined", etc.
Both devices work very well...we've installed both on many clients boats. And, they're "smart"...they won't overcharge your batteries.