Switch 2/Device 2 for the FGD-212 is used for example when you configure the FGD-212 for a double switch config (left paddle dims up/turns on, right paddle dims down/turns off).
See manual page 9 and page 25 at
Fibaro Dimmer 2 (FGD-212) user manual and especially parameter no 20 (page 25) and parameter 26 (page 26).
You can also use both S1 and S2 for scene activation functionality which you can read about on page 27 in the manual (parameter no 28).
On top of it, you can switch S1 to be S2 and vice versa by changing parameter 29 to 1 (default is zero).