Yes, I was going to ask a similar question.
I run a MIDI-shield through Serial1 of a mega, keeping Serial0 for normal use (snip the TX/RX pins and use jumpers to TX1/RX1). I'd like to have two MIDI-ins and was thinking of modifying a copy of the MIDI library so as to run more than one instance with piggy-backed shields or breakout boards.
It isn't so easy for someone of my level to know what to change in the library code, but if anyone could indicate what needs to be done (if it can be done) I could do the work. Obviously, a single multichannel version would be great.
Just one very little thing if Franky47 is still working on it: most MIDI devices send Note Off as a Note On with zero velocity (it's in the standard). At present you have to catch that in your Note On handler; easy, but it would be nice to have it built in to the library. It's always OK to send a proper MIDI off.