I think I answered my own question. It seems that the underlining shows what syllable is blended with the next or previous word. In braille, we show this by using quotation marks, so the underlining is unnecessary. Do I understand this correctly?
So sorry to have taken months – I was never notified about this question!
You are correct, though! The underline shows that the syllables are elided. We enclose the syllables in quotation marks. Then we add dots 12 after the note on which you sing those two syllables.
Thanks for your reply, Kathleen! The curved line indicating syllable elision is typical and very familiar to me but I had never seen the added underlining before.