First, I have requested from the webmaster permission for a sticky (a permanent post at the top of the thread) on basic MIDI and the differences in MIDI/audio. I hope he allows it because most of the MIDI questions around here are usually pretty simple and could be easily answered by a simple sticky. Yours is a little more complicated though.
So you want to be able to:
1. send sound from the MPC to computer
2. send sound from the Triton to computer
3. use the MPC as your main controller
4. use the Triton to control soft synths/soft samplers from the computer
5. use the Triton to control the MPC via midi sequences.
I hope I have that right. Although you said you want to use the MPC as your main controller, you really seem to be using the Triton as your main controller. Maybe I'm missing something here though.
The short answer is that each device has MIDI in/out capability. You can use any device to control another. For example, you could use the Triton to control the MPC and record the MIDI sequence with Pro Tools with a MIDI track. Upon playback in Pro Tools, the MIDI track will playback the MIDI info which will control the MPC. If the MPC's audio out's are connected to the Digi's audio in's, you can record the sound from the MPC to an audio track in Pro Tools as the MIDI track is playing and controlling the MPC.
And the long answer..........I would sugguest connecting everything so that you can control all devices with any device. In other words, you could control the MPC and Pro tools via the Triton, control the Triton and Protools via the MPC, and control the Triton and MPC via Pro Tools. You'll be covered all the way around.
The simplest way I can think of to do that would be to connect:
1. Digi MIDI out 1 to the MPC MIDI in
2. Digi MIDI out 2 to the Triton MIDI in
3. MPC MIDI out 1 to the Triton MIDI thru
4. Triton MIDI out to the Digi MIDI in
5. MPC audio out to the Digi audio in
6. Triton audio out to the Digi audio in
Optionally, and what most people would probably sugguest is to get an additional MIDI interface for your computer and connect the MPC MIDI out to the interface MIDI in rather than going thru the Triton MIDI thru port. You only need 1 additional MIDI in to avoid using the Triton's MIDI thru. Something like the MIDISport would probably be fine. I would hook up the Triton to the Digi first and get that working properly before adding in the MPC or vice versa.
In Pro Tools, you will route the Digi's MIDI in/outs to the MIDI tracks and if your using soft synths/samplers you will need to choose those as your MIDI outs. Also, make sure that any MIDI track you are using is on the same MIDI channel as the device that you are controlling or controlling with.