The Adafruit TinyUSB Library driver for USB MIDI devices properly creates a USB descriptor that can support multiple virtual cables. However, it only provides one MIDI Library MIDI Interface object.
TheMIDInator MIDI controller has been created by Switch & Lever using an Arduino Mega with its 54 digital IO and 16 analog pins to provide a wealth of inputs enabling data to pass onto a digital audio ...
Open the Midi example sketch. There are details at Arduino tutorials. Compile to check the code. It requires no new libraries or add-on files. Setup() turns on the UART circuit with Serial.begin(31250 ...