Just look at the latest RMT fork. Simply use the correct notes and put POKEY's optimal "frequency" generation in. It's that simple. In this tune, I just use 1.79MHz for the "Attack" of the instruments. The rest is 15kHz plus some "filter" Pulse width manipulations. It makes the "Banjo" played on a higher frequency, while the other instrument is played on the standard wave, sounding softer (like a Clarinet).
@Kaz You know the riddle about musical taste :) Well, you also might know that I'm doing MOD conversions. Most of them were free to use and some seem to have some copyright owners. The tune "Fahrstuhl Music" is originally the Highscore tune from Wizball, and YT claims that :D As there no other original exist on the Atari, of course they could be used for any game or demo. If people ask for some...