Why can a bifid manuscript family make it difficult to determine an archetype’s reading?

In the context of textual criticism, a "bifid" stemma (or family tree of manuscripts) refers to a situation where an archetype is represented by only two independent branches or witnesses.1

This structure makes it difficult to determine the archetype’s original reading because, if the two witnesses disagree, there is no third independent source to act as a tie-breaker or to provide corroborating evidence. Consequently, the reading of the source cannot be certified by the agreement of a majority. While the genealogical method allows a critic to reduce the number of possible variants in the archetype to two, the inability to resolve the conflict between those two branches leaves the critic without a definitive path to the original text.1

It is important to note that while the stemmatic method is theoretically sound, its practical application can be complicated by "horizontal" transmission—or contamination—where copyists used multiple exemplars, potentially blurring the lines of these family trees.1