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Question Can I connect two M300 / M350s together by MIDI so the preset numbers stay in sync. Meaning, if I change the preset of the master, is the preset of the slave changed as well

Answer
If you change preset on the first M300 / M350, the second will change preset in sync. However if you send a MIDI Program Change message from a MIDI controller, only the first one will change preset. The M300 / M350 does not merge the MIDI IN to the MIDI OUT.