Co2 regulator question

What is the difference between a single gauge regulator and a dual guage regulator? I am assuming that a dual gauge regulator has a gauge that reads the pressure in the tank and a gauge to read the pressure exiting the tank? Does a single gauge read the pressure inside the tank or exiting the tank? or am I wrong on all accounts? If I add another gauge to a single gauge regulator, does that make it a dual gauge regulator or is there a difference in the inner workings of the regulators? thanks

Single Gauge: reads the "out" pressure (the pressure in the keg(s))
Dual gauge: adds a gauge that reads the "tank pressure" which will remain constant until all of the liquid CO2 has dispersed into gas. The gauge will fluctuate with temperature though, but you should easily be able to tell when the tank is empty by reading it.

Some people say you only need one gauge, some people say you NEED two. It's your preference if you want a warning of when your CO2 is about to be gone. Personally, I went with the two gauge (actually a three gauge, I have a dual regulator)

As far as i know, there is no difference in the inner-workings.