I managed to find a manual for this unit that you speak of. Neat all in one design. I see where the MP3 option comes into play, but is other than working with CD's loaded with MP3's, it's a fairly typical DJ-type set-up, just a consolidated all-in-one design and nice and compact.
So, here's my off-line notes, since I had time today while waitinf for other phone calls, so I copy/paste them here.
Ah, listening to one to get ready to go while the other plays.
This is an interesting configuration and SEEING the unit pictures AND reading the manual makes a lot of sense to me. Also, the logic is a little off as I would think as an audio engineer, but the logic behind it makes perfect sense.
First, we need to look to the middle section of the mixer. Yes, the typical "center master section". Someone used to mix bands! No, it actually makes sense logistically in this environment since it is in between the two playback sections.
1: The MASTER knob. This is your master output playback level, which is the mixed level between the two devices or rather whatever you've routed to the master output. This is the signal that goes out to mains, your QSC gear.
The Master KNOB derives it's signal souce from the "Replaceable Cross-Fader", the one that goes sideways. Full left, it is getting signal from #1 only. Full right gets signal from #2 only. In between, you get your mix between 1 and 2. So, during a song change, you'd move that fader from 1 to 2 or 2 to 1, depending on which way you need to go. This is a "Cross-fade"
This part is very logical. Very straight forward and basic signal routing, which is not only ideal, but appropriate for this environment
The next two knobs in the middle are Cue Volume and Cue Mix.
Think of your CUE volume knob as the "volume for your headphones", which is precisely what it is, because the CUE system is the same as the "control room" in bigger consoles, at least in basic concept. The concept is expanded upon to match the unique environment DJ's are looking at, wihc a regular true mixing console doesn't directly support at least.
The Cue Mix is the main knob you want to focus on for determining what you need to listen to. It works like the "Replacable Cross-Fader" in the fact that it will let you listen to whatever you want independent of what the "Replaceable Cross-Fader" is currently assigned to.
The best way is via example.
Let's say you've got #1 playing, so your "Replacable Cross-Fader" is full to the left and therefore this is the ONLY thing going out your Master OUTS to your mains. Now, you're immediately getting #2 ready to go. Leaving the "Replacable Cross-Fader" alone, twist the Cue Mix to FULL RIGHT/Clockwise which lets you hear #2 only. Bring your Cue Level to say, 3-O'Clock or so(I'm assuming you're gonna need some decent volume and most headphone amps are wimpy). Playing Disc 2, you should hear only disc 2 in headphones. Disc 2 will NOT be playing out of the mains. Should you want to tempo match your songs(I can't but wouldn't anyways, but I'm not a DJ), you can twist your Cue MIX a towards #1(to the left) as necessary so you can hear that along with #2 to see if you're where you want to be. This has NO bearing on what goes out over the mains at all.
Now, this is where setting your CUE points on the players becomes more important. Let's say the song playing on Disc 1 is 3 minutes and 40 seconds long, and it takes you 45 seconds to get things the way you like it on Disc 2. By previously setting your start on Disc 2 via that Cue function(on the player section), you can "snap" back to that cue location. This is cool, at least in my opinion. I can set my "start" point, then go about my business of pitch/tempo matching, then when I'm happy, snap back to my start point with my new settings ready to go, then when I release #2 and cross-fade between #1 and #2, I'm good go to. No "re-cueing". COuld be worse, could be records and having to lift and drop the stylus!
One thing to think of is that just because it says "Cue" doesn't mean the same thing depending on where it is.
Cue on the player means "start here", where as "Cue" in the master section means "monitor/preview for getting the next stuff ready".
Makes sense?
Practice leads to confidence. And by ensuring you understand what's going out where, why and when as you practice now means you won't need to worry about the cue system later on because you'll know how it works. And right now is the time to learn it.