re: two program numbers per channel

JDL tried: sustained note such as trombone with repeated short piano notes
on the same pitch, both on one MIDI channel. How, if possible, can you keep
the piano's note-off commands from also turning off the sustained trombone
note?
Well, shucks. I also tried inserting note-off commands with their own
separate program numbers, but to no avail.
What I wonder is, how m;uch can you edit those things Roland calls "parts"?
My manual's hiding so I don't know how to turn on sysex which would make
this easier. Do ya know offhand what combination of those wee front-panel
buttonlets turns sysex on? Anyway, some of the program numbers obviously
contain two separate happenings, like #86 which runs in parallel 4ths. How
much can one of those notesbe controlled without affeting t'other?
Jay