A user can edit a file he/ she isn't in, if he/ she is referencing the CAD file via the XREF command. He/ she only needs to enable the "redit" command.
As far as working on the same file at the same time, you can't do that. Thinking about... how does the computer know which line takes priority if both wish to work in the same room. I understand you might have users in different areas but the software will interpet the usage as if they were trying to draw in the same space.
Try and split the drawing into 2 files if you're intent is to work in 2 different areas. Then insert one file into the other OR (xref) referencing the other half to each other. This way they remain 2 files but you can plot the files as one.