You will need a 12 vdc (coil) "Form A" relay (form A has contacts that are normally open when deenergized (many relays are Form C, having both NO (normally open) and NC (normally closed contacts - a Form C relay will work too.
Connect one side of the relay coil (on a Bosch style 30/40 amp relay that would be terminal 85 or 86) to ground. The other coil terminal will go to an ignition switched 12 VDC source. Connect one of the relay NO contact (terminal 30 or 87) to ground. The other NO contact terminal will be connected to the ground terminal of the dual voltmeter.