How to notate alternate/secondary melody in ABC

You can come fairly close:

X:0
T:lyrics alignment
K:none
M:none
%%score (V1 V2)
[V:V1] A2 A4 y/8 GF
[V:V2] A2 y/8 A2- AAGF
w: joke kid, __ it's a
w: joke kid, _ for what you

The required code segment is

%%score (V1 V2)
[V:V1] A2 A4 y/8 GF
[V:V2] A2 y/8 A2- AAGF
w: joke kid, __ it's a
w: joke kid, _ for what you

I tried using grace-notes to finesse the small-font notes, but they don't allow for a quarter-note, can't control the stem direction, and can't align lyrics to them.

The answer you seek I believe is here, in section 4.6.3

http://abcnotation.com/wiki/abc:standard:v2.1:proposals:clefs_voice_parameters:v2

Just in case it helps anyone else:

This is called a different 'voice'. The origin of this is harmony (chords) originate in people singing together. Chord literally means in accord - with one another in French. Usually the top voice is 1 and any addtional voices are 2, 3 or etc. By ABC notation do you mean tradition sheet music?

This works on http://www.mandolintab.net/abcconverter.php but not in FiveLineSkink 2.0 Beta, so it might depend on the software you use:

    [| A2 A4- GF &
     A2 A2 AAGF|]

The & is what allows us to express multiple voices.