Ardour's Tracks/Routes are not capable of handing more than one
MIDI port per per route properly. Most Plugin standards don't support
this either.
However, at this point in time IO::ensure_ports_locked() is not
limited by this restriction!
It is still possible to indirectly add a 2nd MIDI data-buffer
and output-port via plugin pin-management, or simply plugins with
two MIDI output ports when using flexible I/O.
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