multiple frequencies for the same key

Hi all
I am improving my tuning program by adding new features, among which the

possibility to use more than one frequency for each key, something like
antique split keys. I am in doubt about the method to represent this
feature on the screen; at present an usual keyboard allows to start
tones by clicking on its keys; I thought to use split keys, up to 5
segments per key, but I fear the segments would appear too small. Can
you wizards of microtuning suggest alternate methods of representation
on the screen?

Another issue. The program internally manages the notes as MIDI codes,
so that future versions can easily connect the tone generators to MIDI
signals coming from outside. MIDI rules assign a code to each key; how
could multiple frequencies for the same key be represented using MIDI
codes?

Thank you

Greetings from Italy

Leonardo Perretti
leo.perretti@projectpp.it