What @Hillbillytnt said. The circuit is simple. The dual voltmeter has two positive terminals, one goes to each battery + through a fuse (for safety, don't wire direct to battery terminal). They don't have to have their own fuses, but can tie in on the device side of another fuse.
The dual voltmeter has a ground terminal, when it's grounded the voltmeter will turn on its display. To avoid having it on all the time, most will use a "switched ground". That is obtained by using a small relay that is energized with + ignition on. With ignition on, the relay is energized, and its contacts apply a ground to the dual voltmeter.
Some will also find it useful to add a momentary button to apply a ground to the dual voltmeter. This allows one to better compare the two battery voltages as they can be checked by pushing the button while the ignition key is off, so the system isn't loading down the main battery during the comparison.