sorry, beginner's question:
in Max4L, is it possible to have a crossfader device, or is this impossible (because of Live's architecture: "audio effect" can only affect one channel, which's input is set in Live's In/Out section).
Not sure what you mean with a crossfader device. ?? Can you be more specific with what your doing and actually want to achieve ?
Just make an empty audio rack (add an EQ 8 , right click create group), set up the chain crossfade region by mapping to a macro, do this on the other track again. Set up crossfades on both racks. Map the same midi knob to both macros and this will create a smooth crossfade. This can be done multiple times. So you can set up as many crossfades as you'd like in a set.
This can be used with audio files dragged in OR with instrument tracks.
I've uploaded a file to demonstrate :)