Lyrics w/ melisma synchronised with linked staves.

I have a piano score here, where the lyrics are sometimes meant to be rhythmically linked to the top stave/right hand, but also sometimes meant to be linked to the bottom/left hand.

I would like to be able to align the lyrics to the union of the two staves (that is, with notes/divisions in either staff), rather than just the top linked stave.
I would like this to include properly extending the melisma to the corresponding note on the bottom stave.

I have the rough score file here, it's public domain (incl. the music), ignore the amateurness of it as I barely know what I'm doing with engraving. The issue is most apparent where I took the screenshot, at the end of the ninth measure.