What you are describing is condensing, which (as of the current version) is mutually exclusive with divisi. If you want S1 and S2 to share a staff, set up the score as SATB and then divide the sopranos where needed.
Divisi is dividing one Section Player (nb: not a Solo Player) into two or more parts, and then joining it back again. Condensing is taking two separate Players and putting them on one Staff.
Condensing is better for instrumental Players, like a bunch of horns, because you need them as separate Parts. With singers, you only have one score, and they don’t use separate Parts.
If you only need one Soprano in one Flow, then exclude S2 from the Flow.