I'll get you through the audio part. Please remember handling the MIDI stuff yourself or ask.
1. Enter Audio MIDI Setup and create a new aggregate device, then add
FIRST Soundflower (to keep the possibility to use the MPC's internal Send FX), then your desired output (doesn't work with all interfaces, unfortunately, though).
2. Select this new device in MPC Software as output.
3. In Reason's prefs, under AUDIO select this aggregate device. You might get some error messages now, but just ignore them. While there, click on ACTIVE OUTPUT CHANNELS and add channels 3 & 4 (or respective).
4. Create an audio track and make it stereo. Now don't forget to route your master or cue out to outputs 3/4 (output 1/2 being Soundflower's output, which'll create an internal feedback loop).
5. If you've done your MIDI sync properly, too, pressing play gets you going nice and smooth:
This works with Soundflower multichannel, too. Only problem is, when adding Soundflower first, you'll max out Reason's 64 outputs, which means not being able to select any physical outputs there. You need to select your physical output first, then route the channels (in MPC Software) to outputs 3+4 / 5+6 etc. and not needing to reroute your master out in Reason, but this way you won't either able MPC's output 1/2 or using MPC's returns, submixes and / or master FX.
Enjoy!
