Hi Ducati
The ideal is to have both gauges hooked up. The voltmeter is a cinch, just find a live circuit to hook into when the ignition is on. The minus on the gauge gets hooked into any convenient ground.
Ammeters are great because they tell you how much current you are using, and how effective the battery's condition is. IOW, if the battery is almost dead or dying, the ammeter will show a strong charge condition, instead of the needle being in the middle. The ammeter looks and acts different than a voltmeter in that "normal" condition is in the middle of the gauge. (Look at an old Triumph moto, on the headlight shell)
A drawback to an ammeter is that you must pull the big hot wire coming from the alternator (which goes to the battery) and route it through the ammeter gauge, then to the battery. A minor drawback actually, but worth the hassle. You just need to make sure that the connections on the back of the gauge don't ground out on something, because you will get full battery zap!
I think I will look into a airplane gauge, it sounds like it is two gauges in one...?