Varying instruments in the system

I want to have measures with a single staff, then measures with a grand staff, then back to a single staff. I tried this by selecting measures and changing in Edit->Instruments accordingly, but that affected the whole piece. I also tried to do section breaks for the areas I wanted these differences, but it gave the same result -- the changes in Edit->Instruments applied to the whole piece.

A friend has done this in Sibelius, it seems. How do I do this?