http://www.youtube.com/watch?v=fcdcWgDi9cE < here is a video of this function.
I watched the vid and thought it was pretty sweet so I had a crack. I think I've had reasonable success achieving this and thought some of you might be interested.
This one is 8 band so it's pretty smooth, can almost sound like a frequency sweep type effect if you want. You might like to modify the design to have less bands if you want - I think the djm2000 is 7 bands.
You will need 9 spare remote midi CCs to run it. They have to be remote control rather than 'script' control (APC normal mode etc) as it requires co-responding macros across your two x-fade channels to be controlled simultaneously. I guess otherwise you could sync them with MAX but I can't be fucked doing that myself at this point hehe.
Anyway here are the racks for download > http://www.sendspace.com/file/nvvdo9
It's basically an oppositely mapped pair of 8 transparent (relatively) filter x-overs for each track with various utilities devices to keep the gains in check during transitions. (I have used ethios4 sweepable crossover rack idea in this, thanks mate)). There is also a macro on each rack to assign to your regular X-fader so that you can use both regular X-fading and 8-band simultaneously
instructions:
- Put racks '8-band with Dry-Wet A' and '8-band with Dry-Wet B' respectively on the two tracks you wish to x-fade between. If you plan to x-fade between multiple tracks it might be easiest to bus them - up to you.
- Set the A and B x-fade switches for these tracks as you would normally.
- Midi-map your X-fade controller to the ableton X-fader as you would normally
- Midi-map your X-fade controller the macro 'X-fade control A' on the device '8-band with Dry-Wet A' - you must invert the range for this mapping.
- Midi-map your X-fade controller the macro 'X-fade control B' on the device '8-band with Dry-Wet B'- do not invert the range for this mapping.
- assign your chosen 8 frequency knobs/faders to the 8 macros on both devices. You do not need to invert the ranges for any of them as this is built into the macros already.
How to use it.
When all your 8 frequency x-fade knobs are set to the middle values, the X-fader will behave like a regular X-fader.
While the X-fader is in the middle of it's range, the 9 frequency band x-faders are active. Turning them to 0 will result in all of track 100% A and 0% track B for that frequency band. Turning them to 128 will result in 0% track A and 100% track B.
The main X-fader control will always play 100% of track A or B at either extreme regardless of the current state of the 8 frequency bands - it over rides them.
Here are some quick diagrams that might make a bit more sense than that waffleing.
Blue and Red bars represent the gain for each of the 8 frequency bands on either track.
^^^^frequency band knobs have no effect when X-fader is at either left or right extreme
^^^^ both X-fader and all frequency knobs are centered - x-fader behaves like a regular x-fader
^^^^ while x-fader is in center (approximately), 8 filter knobs can be used as individual x-faders.
^^^ as the x-fader moves from center to the far extreme, 8 frequency bands are mixed with the dry signal of the track,
their effect becomes less pronounced.
^^^^ all frequency bands have been move to 100% on the left while x-fader is centered,
essentially same result as if the X-fader where 100% to the left.
Due to limitations in ableton's racks I couldn't quite iron out one the gain transitions. The problem occurs when you are fading between the situation in the last picture and the first picture, resulting in a breinf drop in gain. Once you get familiar with it to it its just a matter of quickly moving between the two and it is not noticeable. There would be no need for you to fade between these two slowly anyway as they are have the same result.
If anyone can dive into the rack and tweak the macros to prevent this from happening and provide an update, go nuts!
Also, I'm sure with MAX you could link the macros from the two racks. This would mean you could control them from regular control surface script device control, rather than hard wired remote mapping.
Tip* It might be a good idea to have a midi clip set to center the frequency knobs via IAC drivers or midi yoke, and assign the launching of this this to a button. That way its nice and easy for you to center the frequencies when you're done with your fade. Otherwise you could just map a button to turn both devices off and regular x-fadding will be functional as normal, by-passing your frequency settings.
Another tip* One cool thing I was doing was setting the frequency knobs in a pattern and then just fading across from left to right - the result is that some frequencies start to fade across early and some late. If you stagger them from high to low it sounds like nice smooth crossover sweep like effect.
and the patch: 
)
I still need to modify the CPfade device further; in short the crossfade power levels need to be different depending on whether I am performing a fade between tracks or between effects on/off. At some point I will get this all finished & cleaned up to share, but for now I am off on vacation.