Combining lyrics lines

[Reedmace Star]

Dear Lilypond users,

I am typesetting a musical-theater style song, with the parts of several 
singers appearing on the same staff.

For unrelated reasons, I want to use a separate Voice context for each 
singer, even though the melodies usually don't overlap. The lyrics are 
defined separately for each voice, too.

Here is a simplified example of what I'm doing:


\version "2.14.2"

<<
  \new Staff <<
    \new Voice = "a" { f'4 g' r2 | s1 }
    \new Voice = "b" { s1        | a'2 g' }
  >>
  \new Lyrics \lyricsto "a" \lyricmode { may -- be }
  \new Lyrics \lyricsto "b" \lyricmode { that the }
>>


Now, the lyrics to "a" and "b" appear on separate lines, like this:

     maybe
             that the

Instead, I would like to see them combined on one line whereever 
possible, like this:

     maybe   that the

Lilypond 2.12.3 did this by default. Is there a way to achieve it in 
newer versions?


As mentioned before, the melodies usually don't overlap, so it would 
probably be sufficient to overlay both Lyrics contexts in the same 
vertical space.  I naively attempted to do this by putting both into a 
StaffGroup and then setting their Y-extents to zero, but this had no 
effect; neither had various experiments with the staff-staff-spacing 
property of the StaffGroup or the Lyrics contexts.


I also considered using a single Lyrics context and associatedVoice:


\version "2.14.2"

<<
  \new Staff <<
    \new Voice = "a" { f'4 g' r2 | s1 }
    \new Voice = "b" { s1        | a'2 g' }
  >>
  \new Lyrics \lyricsto "a" {
          \lyricmode { may -- be }
          \set associatedVoice = "b"
          \lyricmode { that the }
  }
>>


This fails, I guess, because of the requirement to change the associated 
voice one syllable early (2.1.1)?  While that would be easy to fix in 
this minimal example, it would be inconvenient in the actual score, 
where the lyrics are stored in separate variables defined outside the 
\score block.


Any other ideas?


Thanks and best regards,

Reedmace Star