Yup, that's what I'd do with one exception. Wouldn't use that style of relay. One of the 30A or 40A cube-style relays (Bosch, Hella, etc.) would be more than adequate to feed the second battery.
Oh, and don't forget fuses first in line off your "hot" sources.
That said, I do wonder what the effects might be of the second battery backfeeding the starter circuit. Not concerned about the starter, just about the relay, fuses, etc. with the starting current draw. Would be easy enough to put a diode inline though to prevent that backfeed. Diodes though drop voltage and are less efficient than the relay method.
Suppose could use two relays in series. One that closes when in ON/RUN position and one that opens when you hit START. Or, we could assume relatively safely, I think, it's not going to be a problem and just use the single relay.
Last edited by whoaru99; 11-04-2012 at 09:24 AM.