There are different ways to represent multi-voice music in ABC and not all of them are portable. The two basic approaches are to enter the voices in separate complete chunks (as Jon did) or to match them up line by line as in the staff notation (which is what I do - it's more likely to work on a range of implementations). You shouldn't need to restate key, tempo and default note length for each voice as Jon did.

Have a look at the "tunes with a bass" and duet sections in my "Old Scottish Flute Music" pages.