Two syllables sung on a single note need an elision mark, not two stacked lyric strings

When two syllables of text are sung on one note (e.g. French/Italian text or an English contraction sung as one note), an elision mark — a small under-slur or undertie glyph — joins them, and they are not stacked vertically or run together without a marker.

A note is authored with concatenated lyric text with no elision glyph, or two separate lyric entries are both assigned to the same note index with no visual link, producing ambiguous or doubled text under one note.

Gould Elision; Read (undertie convention); MusicXML lyric/elision child, default undertie glyph U+203F.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.