Usually when I need split staves in some place and combined staves in others,
I use a score with 3 staves: one for each single staff, and then one combined staff.
For the pages where you want combined, both single staves are empty.
And for the pages where you want split staves, the combined staves are empty.
Then use \RemoveEmptyStaves to get rid of the empty staves
The issue with this approach in your case is that
you want the combined staff to show only when there are rests,
so in order to get those to print, you would need to add something
to the combined staff so that it was not empty.
I tend to use an invisible dynamic like \tweak stencil ##f \mp
HTH,
Elaine Alt
415 . 341 .4954 "Confusion is highly underrated"
elaine@flaminghakama.com-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-