Mostly unisono multi-voice staff also merging whole notes together

[Hans Aikema]

Hi all,

In a score I have two parts (Alto I and II) that’s for most of the score 
unisono.

In order to be able to also generate rehearsal midis by voice (with the 
concerned voice highlighted by diminishing the volume of other voices) I would 
like to retain the full individual voices.
This rules out 
… << { \voiceOne … } \new Voice { \voiceTwo … } >> \oneVoice ...
pattern for temporal polyphonic music as it introduces the problem that the two 
voices displayed in the staff are now split in a way that would at least not 
allow me to create a rehearsal miditrack with voice that takes its polyphony 
notes from the \voiceTwo section highlighted.

For easier readability of the unisono part however I would like (visually more 
condensed) the whole notes to join up like all the 
identically-headed-and-pitched stemmed notes do.

So what I’m trying to achieve is the notation effects of 
<<
    { \once\override NoteColumn.ignore-collision = ##t f'1 | \once\override 
NoteColumn.ignore-collision = ##t f'1~ | f'1 | f'2( g'4 a'4) } \\ { f'1 | f'1~ 
| f'4( e' d' c') | f'4( e' g' a') }
>>

% rather than the fully proper from notational perspective default rendering
<<
    { f'1 | f'1~ | f'1 | f'2( g'4 a'4) } \\ { f'1 | f'1~ | f'4( e' d' c') | 
f'4( e' g' a') }
>>

% or the completely broken
<<
    \override NoteColumn.ignore-collision = ##t
    { f'1 | f'1~ | f'1 | f'2( g'4 a'4) } \\ { f'1 | f'1~ | f'4( e' d' c') | 
f'4( e' g' a') }
>>

without having to sift through the score looking for the colliding whole notes 
in order to add a \once\override NoteColumn.ignore-collision = ##t to them

Wondering if there would be a good solution that I’m somehow overlooking

kind regards,
Hans Aikema