Split Staff for A/B Parts During Repeat

If I understand correctly, you are talking about a repeated passage of some length but for some a portion of that passage, the music is different? This is only supported - in standard music notation and MuseScore - through voltas as endings. No doubt some composers experimented with an alternate style of notation in which multiple staves are used to mimic the effect of voltas somewhere other than the ending, but it's not standard notation, and not supported in MuseScore. You can certain add the second staff, and set it to hide when empty and "cutaway" so it only shows where needed. But the playback won't understand this non-standard notation - it would just play both versions at once (although you could silecne one or the other. if you need to generate an audio file, you could use Tools / Unroll Repeats and then fix it up tin the unrolled copy.