Each virtual ports is its own MIDI cable, so it can host 16 MIDI channel, so each virtual port can be used by at least one MPE controller (input) and one MPE synth (output). I say “at least” since you can always reduce the number of member channels, and for example having 1–8 to synth 1 and 9..16 to synth 2, given it will have a potential effect on the available polyphony.
That said, to use the output virtual port, you either need a synth that understands virtual ports itself at its USB MIDI input, or, like I do currently, use a MIDI HUB that does that for you.
I can only talk about the MRCC because this is what I have for that, but I know the mioXL is a great Hub.
So on the MRCC you have 4 USB Host ports. Each can host 4 virtual ports (1 to 4). So I plug my Hapax from its USB device port to one of the MRCC USB ports. It means I will have 4 virtual ports exposed and available to the MRCC on each USB ports (Ports 1 to 4). On the Hapax, I set 1 track to use MIDI Output USB Device 1, another one MIDI Output USB Device 2, and so fort up to 4.
ON the MRCC I route each port to a distinct MIDI Output Destination, usually running a simple MIDI DIN channel.
On the controller side, there can be different strategies. Mine is to plug an MPE Controller from its USB device output port to another free USB host port on the MRCC. Typically, I use up to three MPE controller (one ERAE2 and 2 ROLI keyboard). Each is routed to the same USB Host port I talked above (the one the Hapax is connected to), back to the Hapax (since USB is bidirectional).
ON the Hapax, on each track, I select from which port I want to listen, i.e., which controller, to play the synth on this track.
This allows me either to play directly a MPE synth live, and/or record the notes on the Hapax.
Note: I didn’t fully tested everything yet in this setup, it’s a work in progress. But theoretically, and potential bugs apart, it should work and is pretty flexible. It’s easy to change settings on the Hapax. And on the MRCC you can recall various setup easily. I’m also super interested to hear about others setup and strategies.