Like Todd said, you only use one cue button.
If you mix in your cans, you use the cue/master knob to set the level so that both the incoming (cued) track and the playing sound (master) have the right ratio. Some like one louder than the other, some like them both the same level. But that is personal choice and you can set it here. Only the incoming track is on cue.
The following scenario is possible:
1) you have the mentioned knob set somewhere between cue and master
2) you have both cue buttons pressed
3) you now hear both tracks at full level PLUS the master sound to an extend
4) when you start mixing, the playing track will become less in the master sound and the new track will go up. This changes the dynamic of what you hear in your cans, while still playing both tracks at full strength (thanks to both cues being on).
If you use only one cue button, the sound from the cued track will remain high, but the master sound will become increasingly more the new track. By the end of the mix you will only hear the new track in your ears, but really twice, once as cued track at full strength, once as master output.
I am assuming that you mean the X-faders is on THRU not TRUE, meaning it’s effectively not doing anything and you use the channel faders for mixing. So that can be taken out of the equation.
Hope that made sense.
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