Way back in the day, Opcode’s “Studio Vision” had this great feature. You could select two or more instruments and layer them and save as a new, easy to recall instrument. It would save presets for each and you could assign zones, transpose, crossfades, all kinds of layers, set volume levels etc. I had mega string instruments, cool synth layers, layers and pick and choose drum sets etc
Seems simple enough to do today with VSTs.