The empeg-as-a-DJ-device has been discussed on this BBS before. You wouldn't need two sets of pre-outs to do it. All you need is to do the mixing in software, using the existing sets of outputs.
The only problem is that the empeg folks are busy working on projects that are more consumer-oriented, and are more related to the product's core target market. A DJ unit would be a pretty specialized device and wouldn't sell a large number of units.
I agree that the empeg car player is very very close to being the perfect DJ device already. It's the perfect piece of hardware and it's just ripe for doing what you want.
I can see two ways you can do what you want:
1) You could write the DJ software yourself. The specs to read the files and output to the audio hardware are known. It wouldn't be an easy undertaking, though. There are thousands of man-hours rolled into their current player software already, and the only way to do it yourself would be to re-invent the wheel.
2) I just thought of this: Just run
two empegs into a mixer. Except for the speed control, that would totally fulfill your requirements, wouldn't it? And if you pre-processed the audio files to fall into a specific BPM range, then you wouldn't need the speed control (unless I'm misunderstanding how DJs do their jobs).
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Tony Fabris