I'm assuming your IEMs have two separate inputs where you can feed Aux 5 to the Left channel and Aux 6 to the right channel. At no point should your Aux 6 provide anything to the Left channel and Aux 5 shouldn't be able to feed the right channel. If you don't feed different left and right sounds, it would be hard to notice the difference. (Use a stereo recording for testing feeding the left into channel 1 and the right into channel 2 (Linked and fed out to Aux 5/6 link.) Follow the signal through the mixer. It should work. (It does on mine, but I only played with stereo out. Don't use it as it doesn't meet my needs.) If you do have a problem with this, you might need to reset the mixer and reload the firmware. Trace it down and check the signal at every point through the headphones. Which mixer and which version of Master Fader are you using?