Not sure if you got an answer, but you cannot break up a 'top-level' sheet into multiple schematics. In other words, you can have only one single root / top level sheet.
What you have to do instead is instantiate hierarchical sheets, which are sub-sheets under the root sheet. This somewhat forces you to 'block diagram' your circuit, and define the hierarchy beforehand so this works out correctly.
I *hated* this to start off with (especially since it's different than nearly all other CAD programs), however you end up with a usable global-root sheet that always completely describes your circuit on an entire single sheet.