If I’ve understood you right, this is fairly straightforward using extra players rather than divisi. Have a look at the attached.
For completeness here are some related thoughts/tweaks. As an old hand, I’m sure these won’t be new to you
I changed the name of Soprano II to " Soprano" [note the added space] so that Choir I sop would have staff label “Soprano” [rather than Soprano I]. Other players similarly.
When Choir II was singing I sometimes had to add shift-X [i.e. invisible text] to a rest in an otherwise empty whole stave to make it show up (Galley View is useful here).
When it changed to two choirs I used shift-X to add “CHOIR 1” and “CHOIR 2” above the respective choirs.
In another flow you could make a further set of extra players if that helped, for example if you wanted two choirs to show throughout the flow with no hidden staves. Obviously set which players you want to show in which flow in Setup Mode.
You could use Solo players for either or both choirs rather than Section players if that gives you other options that you’re after.
Hope this helps.