Hi Luis. If your external keyboard has only USB MIDI, yes that's impossible. If it also has 5-DIN MIDI out, then it's possible.
The new OS has not addressed this issue, but then, it has these new custom scripts, which may actually allow multiple MIDI devices, as they allow us to go deeper in the system/OS (such as allowing Wi-Fi connection).
I have already asked if the custom scripts will allow multiple MIDI devices. Keep watching the new OS thread here on the forum.