Overlapping ties in different voices

[Frederick Bartlett]

Hi!

I'm trying to set selection from Saint-Saën's Oratorio de Noël for my choir.
In the Gloria in altissimis (p9 in the IMSLP PDFs), I found an odd combination of ties:

\relative c''' {
  << {           s1      | b1 ~                  | b2 cis  } \\
     {           a1 ~    | a2 s2                 | s1      } \\
     { \voiceTwo r2 a, ~ | a <g' g,>4 <fis fis,> | <g g,>1 }
  >>
}

This peculiar mess is almost right; I just need to figure out how to keep the b and a on the first beat of the second measure from colliding -- but no combination of \shiftOxxx and \once \override NoteColumn.force-hshift that I've tried has answered the case. Leaving the \voiceTwo command out prevents the collision -- but at the cost of getting many other things wrong.

To match the 19th-century original, the b should be shifted a notehead's width right.

Help?

Thanks!
Fred