The two inputs of EVO 4 can be stereo linked which allows you to match the gain for both channels to make sure you're left and right channels are at the same level.
This would be used when recording an instrument using two microphones, such as a piano or guitar, or if inputting a stereo line signal like a synth or DJ decks.
To do this, press down the 1 and 2 buttons at the same time, now changing the gain for one channel will also set the gain for the other channel.
Smart Gain also works with a stereo linked channel.