I never did take the time to see how more than one voice was done in ABC's, but I spent a couple minutes browsing The Session today... there it was - staring me in the face!

Geez, it does pay to not overthink.  

The body of the tune ABC's goes like this... 

V:1  

this next line of ABC's is the 1st line of the 1st voice 

V:2  

this is the 1st line of the 2nd voice  

V:1  

the 2nd line of the 1st voice  

V:2  

the 2nd line of the 2nd voice  

V:1  

more 1st voice, etc...  

V:2  

more 2nd voice, etc...  

The 'V:1', 'V:2' repeat (with the ABC's following on the next line).  You could easily add 'V:3' I imagine, BUT...  

THIS IS FOR HARMONY where BOTH lines are in the SAME clef, the SAME key, etc...  

Now, I know I've run across tunes in ABC Grand Staff - unfortunately, I wasn't interested in ABC's at the time!  ...need to look for one later, to see how much more info needs to be added.

The Session discussion I mentioned shows a little dissonance in a wonderful example of counterpoint (polyphony) - 'The Green Mountain' played with 'The Maid Behind The Bar'!