Is there a way to split an (eg) Violin 1 section temporarily into 2 staves for a section where there is a solo as well as a TUTTI and then to revert back to one stave when the solo is finished?
I’m afraid not yet, no, but this is definitely something we plan to add.
When this gets implemented it will be a milestone in the world of music notation software! 
Until that happens it’s certainly possible to “work around” the scenario. One can either add a (solo) instrument to the Violin (Player) and the solo line will appear as necessary automatically, or by simply adding a solo player, which also will appear as necessary provided empty staves are set to hide. The attached shows both approches using 2 flows.
solo+tutti.rar (1.82 MB)
And behold, Dorico’s Divisi feature did indeed get released in version 2.0, back in 2018.