I have recently created a small jesusonic plugin called “MIDI copy to channels” to facilitate sound layering with multitimbral synths.

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Basically, it allows you, with one midi source, make several sounds played. In my case, I find it rather useful with synths like TTS-1 and Roland Sound Canvas.

Feel free to get the plugin here: https://github.com/hq9000/reaper_tools/blob/master/js/midi_copy_to_channels.js

More on that in the video below.