That is well described by Craig:
http://forum.cakewalk.com/FindPost/3381201 From myself I can add that using the same hardware knob for ACT and for that "remote control" is a bad idea. I mean you should not assign the control to ACT MIDI (Generic Surface, A-Pro, etc) and then try to use it as MIDI control (either for synth rack or for any other purpose).
Most controllers have more then one hardware preset/program/mode which can be switched fast, so you can configure to send different MIDI messages in different mode. Then you can use one mode for "ACT" and another for "MIDI learn". Also note the the later can also leak to the MIDI tracks (they are not filtered out, unlike ACT controls).