Reports using Multiple Cubes

What you are describing sounds like vertical extensibility, where the detail cube(s) consolidates into a main summarized cube.  If you have set the Entity dim, and cubes up properly for vertical extensibility where OS out of the box consolidation is summarizing the data to the parent entities in the summary cube, then you simply point the Cube View to the parent cube in the POV, and OS will auto-magically transition from summary cube to detail cubes relative to the entities.

If you are not using OS's vertical extensibility to link the detail cube(s) to the summary cube, and are using your own logic or some other process to copy summarized data to the summary cube, then no, you cannot have a single Cube View reference multiple cubes.  You will notice you cannot select Cube in the rows or columns, as you can dimensions.  Though could specify a cube override in the row or column expansions, the Cube View is still going to use the dimensionality of the Cube specified in the POV.