Multi-voice ledger-line legibility budget is shared, not per-voice

In a texture where two or more voices share a stave (chorale SATB, second-species stacked counterpoint), a passage where BOTH voices simultaneously reach into ledger-line territory compounds legibility cost beyond what either voice alone would cost — two sets of ledger lines interleaved on one stave, on opposite sides of the shared noteheads, is harder to read than either voice's excursion in isolation. The sustained-passage threshold (ledger-line-sustained-threshold) should therefore be evaluated per-voice AND with a stricter combined threshold when two voices are simultaneously ledgered.

An SATB passage where the soprano sits 2 ledger lines above the treble staff AND the alto simultaneously sits at the bottom of the treble staff needing its own ledger line below middle C, for several consecutive beats — individually tolerable, jointly a genuinely cluttered stave.

Gould, Behind Bars, 'Voices on One Staff' chapter, discusses multi-voice ledger-line crowding as a distinct aggravation of the general leger-line sparseness rule, though she gives no separate numeric threshold — this is Gradus's own extension, reasoned from her general principle plus the compounding collision surface documented above (ledger-line-collision-with-adjacent-voice-notehead).

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