Hey Bus Driver - here's the deal: The two unit (two relay) voltage regulators are arranged so the "voltage regulator" relay also senses and limits output current. It does this with a low impedance (a few turns of heavy wire) coil wound outside the high impedance (many turns of fine wire) voltage sensing coil. The current coil is wired in series with the generator armature and sees all the generator output current. Now when output current exceeds the regulator, it pulls the regulator points open, dumping the field, thus reducing generator output voltage and hence current. In this photo of a 2 element regulator (from an SH), the regulator relay is the one closest to the camera. The current winding is the heavy copper coil wound outside the voltage sensing coil.
The two relay setup is a cheap, but imperfect solution. Adjusting the voltage setting affects the maximum current setting and vice-versa - ie. they are very difficult to adjust properly. Thus the more common 3 element regulators with separate relays for voltage, current and cutout control - while more costly, they do a more precise job of generator control.
Hope this makes senses! ...Bob M