I was recently working on a project to do this, and more.  Short answer is
that what you want to do should be easy.  Volt meter, and a single pole
double throw switch.  This will not join the batteries at any time, so you
don't have to worry about further isolation.  I want to use a cheap digital
multimeter to do what you have said.  I also want to measure amps going
into or out of my aux batt, and 120 VAC output of my inverter.  I took
apart the meter, and found I would need some really fancy switch to be able
to press a button and have it a) connect the meter to the appropriate wires
in the van and b) set the meter to the correct input setting.
Edward

At 04:57 PM 2/19/2003, you wrote:
>I have somewhat limited real estate for installing gauges in
>the dash. I was wondering about the feasability of using one
>voltmeter (with a switch) to read the voltage in two
>batteries. One is an Optima Deep Cyle, and one is my standard
>starter battery. Any reason this wouldn't work, or would
>something about the wiring required remove the isolation
>between the two bateries? I am using the "silver fridge
>relay" setup from www.vanagon.com
>
>Thanks,
>Chris