Doable - yes. But I wonder about the use case. 2 users sharing books?
I wonder how did you get "copy and rescan" without creating ".noscan" file? ^^
Regarding the ".noscan" file, one of the easy ways is opening notepad and saving the file with type "any file" (or whatever it's called) and the given name.
Removing & adding card is not nicely "supported" but should never get you stuck somewhere. In the worst case (which should never happen) just press and hold "menu" button, it should get you back to the root menu.