How to show only one staff before a divisi on a system?
Dorico doesn’t currently provide this option: it will always show a unison range preceding the start of the division on the system.
To be fair, it is rather rare (up until quite recently) to find published scores where a divisi is indicated in this way (the staff suddenly splitting mid-system). It makes for awkward reading.
In general, the first measures of a staff, until the divisi, will show two parts in unison. The same goes for a return to unison after a divisi passage.