Only one input should be used per time, though both 3.5mm and 2.5mm headphone connections are always active and can be used in parallel (though different headphones will naturally have different output at the same volume settings.) unbalanced signals will be internally converted to balanced.
For IEM’s (especially very sensitive pairs) that show up every little noise use the 3.5mm S-Balanced connection or add the 2.5mm iEMatch.
