Apollo should be able to put two instruments inside of a parens to have them start at the same time.
For example
((1/4 "guitar" C D E) (1/8 "piano" C D C D))
should have both the guitar and piano play a C note at time 0, it should have the piano play a D note at 1/8 seconds, and have the guitar and piano play a D and C at 1/4 seconds, and then the piano play another D at 3/8 seconds, and the guitar play at 1/2 seconds into the song.
Then any other notes after should continue at 3/4 seconds.
Some other notes: The quotes should not be necessary around the instruments, and there should be documentation for all the instruments that contains the function used to generate them and the wave sample for each note on the instrument.