Opposing stems on a shared stave must not visually collide or overlap when their notes sit close together — stem length may need extending, not just fixed direction

When two voices' noteheads sit close in pitch (a third or closer) with opposing stem directions, the default stem length (typically one octave, i.e. 3.5 staff spaces) can leave the two stems' free ends nearly touching or the beams/flags at those ends overlapping. Standard engraving practice extends the stem of one or both notes slightly beyond the default length so the two stems, and anything attached to their ends (flags, beams, articulation marks), keep clear of each other.

An up-stem note and a down-stem note a third apart on a shared stave, both at VexFlow's default stem length, whose stem ends (or attached flags, in an unbeamed passage) overlap or touch, producing an ambiguous tangle at exactly the point a reader's eye would otherwise use to disambiguate the two voices.

Gould, Behind Bars, and Ross, The Art of Music Engraving, both note that stem length in multi-voice writing is adjusted case-by-case for clearance rather than held to one universal number — this is one of the few conventions where the authorities explicitly decline to give a fixed figure, deferring to 'whatever clears the collision' as the rule itself.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.