Better yet, use the new dual-battery setup that West Marine recommends... it's in their diagrams in their catalog. It uses three single-pole switches about $10 each, instead of the usual off-one-both switch... you use a shutoff switch for each battery, and a switch between both batteries to isolate them or switch them into parallel in an emergency.

I wired my boat up this way this past spring, and it works great...

Battery #1 has its own shutoff switch #1, is used to start the main motor and is charged from the main motor, and that's it.
Battery #2 has its own shutoff switch #2, is used to start the kicker motor and run all of the electronics and that's it.
If either battery becomes drained or has a problem, you turn switch #3 into the "on" position and it puts both batteries in parallel so you can start the motors or run the electronics... pretty cool.

Remember, battery isolators are diodes and as such, have about a 1v voltage drop across them...

-N.