Rest vertical placement on a shared stave follows the same voice-separation axis as the forced stems, computed correctly

On a shared stave with two voices, rests for each voice are displaced to opposite sides of the staff's center (upper voice's rests higher, lower voice's rests lower) so simultaneous rests in both voices don't collide — mirroring, but logically independent of, the stem-direction convention.

Two simultaneous rests in a shared-stave voice pair rendered at the same vertical position, reading as one rest rather than two independent voices both resting.

Gould, Behind Bars, voice-pairing section (rest displacement on shared staves) — a companion convention to, not a consequence of, the stem-direction rule, since rests have no stem to hang the logic on.

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