Don't want to make a habit of answering my own posts, but I found a utility called Polymer (macOS) that seems to be able to do exactly what I had in mind, i.e. I would be able to set midi channels and devices such as: MatrixBrute keyboard plays MatrixBrute up to 3 held notes (in Paraphonic Mode), next two notes get sent to another synth (my putative Neutron) so I get a paraphonic 5-note analog monster.