I think most people on this thread are talking about two different things. On my djm-600 there is a switch to allow you to change back and forth between listening to your live track in one ear and your cue track in the other, vs listening to both tracks in both ears and using the analog dial to switch between percentages of live and cue. I personally cannot beatmatch when I have one track playing in one cup and the other playing in the other. I can however, and do, transition with only one cup over my ear. The cup has about 75% of the cue track and about 25% of the live track so that I can tell if they are getting out of beat in my headphone. I listen with the other ear though, to the monitors, because that is what is actually playing live and that is how I know if I'm screwing up or not--because the only way to hear what the crowd is hearing is to take your headphones off!

Most mixers allow you to listen to both tracks in both ears at the same time. As far as I know, only a few mixers allow you to listen to one track in each ear through your headphones. When professional djs are only using one cup, chances are they are still monitoring the beatmatching and listening to BOTH tracks in the one cup that they have on, and are listening to the monitors to hear what the crowd is hearing.

actually most djs dont have both tracks in one cup
they use one cup..with one track..and the monitors like it is suppost to be
some may do this though
beat match like normal. and if they arent sure..then mix to cue to the master..and preview if it is on
personally the only time i do this is if im going from trance to breaks

I'm sorry, I didn't realize that you were most djs Anyway, it is the easiest to beatmatch while listening to both tracks in one cup, so I assumed that the pro djs have already figured this one out. Maybe I'll set a pole on it to find out what "most" djs do

sorry..didnt mean to speak for most djs..
but if your talking about international and national talent.
i work at a club a club..and dj there..but on my off nights i run the ligts..so i am able to see what ever dj does..and i have yet to see one dj do that yet
mike

I agree with MigityMike. I've used to use both headphones on at the same time method. But I know a couple of guys that once they don't have that option on a mixer they're screwed. For instance my friend has a Vestax PMC170A, and on this you can't adjust the volume of the cue track in relation to the one that's playing.

It takes a while to get used to, but once you train your ear, you can beatmatch just as well.

I personally preffer this method as you can always use use it no matter what mixer you have. Which is good for those of us that started with crappy mixers.