Whole notes have no stem at all — a same-pitch, same-duration whole-note unison across two voices is the one case where the notehead offset is the reader's ONLY cue that two voices exist, and Gradus does not apply it

For any beamed or stemmed duration, an opposing-stem unison (see opposed-stem-unison-same-duration) still gives the reader a fallback cue even when noteheads coincide exactly, because the two stems point in different directions and can be told apart by that alone. A whole note (`w`, `w.`, `w..`) has NO stem in traditional notation — VexFlow renders it as a bare open oval. If two voices share a stave and both sound the identical pitch as a whole note in the same bar, there is no stem, no flag, and (per opposed-stem-unison-same-duration's current, unfixed state) no offset either — nothing at all distinguishes 'two voices happen to coincide' from 'this is simply one note.' The reader has no way to know a second voice is even present.

A whole-note unison (identical pitch, identical `w`/`w.`/`w..` duration) across two voices sharing a stave, rendered as a single unshifted oval with no accompanying stem, flag, or marking of any kind — structurally indistinguishable from a single voice holding that note, when in fact two independent voices are sounding it.

Gould, Behind Bars, and Ross, The Art of Music Engraving — both note that whole/breve notation removes the stem-based disambiguation available to every other duration, which is precisely why the notehead-offset (or an explicit 'a2'/'unis.' marking, see a-due-unison-merge-notation) becomes MANDATORY rather than optional for this duration class specifically.

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