Issue when converting a MusicXML file to MIDI

When I imported a MusicXML file into MuseScore, the music notation looks
just like the original score:

I then output this to MIDI. When I look at the MIDI score on my Yamaha Clavinova, all the notes are on the treble clef assigned to a single instrument instead of separating the treble and bass clefs:

How to I get MuseScore to produce a MIDI file that separates the treble and bass clefs, assigning them to two instruments?