Proprietary drivers have the bad habit of not being updated when the manufacturer goes out of business or when it has no more economic interest to support an old out of production product. OR you could offer 2 ports on a single midi cable with a proprietary driver (for the very few who actually need 2 ports). If you really need 2, then a second class compliant usb cable is an option.

So in my opinion a single class compliant midi port is the way to go.

And anyway you will be limited by the sound generator polyphony (notes drop out) or DSP effects (audio quality). And the result is crowded enough that you really do not need more.

Our software ( on the other hand sends up to 16 channels altogether (8 accompaniment parts, 4 parts from the first manual, 4 parts from the second manual + pedalboard). When all the recordings are made, the final mix down is done into the DAW with all the bells and whistles it can offer (including a full stack of effects applicable separately to each channel). To say it all, probably the few using the Genos & Co as an expander will only send a single channel at a time (and record back the audio into a DAW). Who realistically needs 32 input midi channels (= a jam of 32 different instruments) mixed together by the keyboard into a single audio stream? Especially if you consider that Genos & Co can only mix down all the incoming midi channels into a single analog audio stream, output trough the onboard speakers and/or analog audio jack. In this use case 16 midi channels are usually more than enough. The midi in is needed when you use the arranger as a sound module ('expander') driven by a PC (sequencer). In this use case midi in/out is never used. Yamaha arrangers have 2 midi ports in order to allow to reproduce a midi file with the internal midi player (that could saturate 16 channels) and at the same time allow the user to play in real time above the midi file playback (another few midi channels needed) using the built-in keyboard.

Yamaha usb midi driver