Multisite subdirectory folders?

Hi
Is there a way to use multisite to get the following structure:

Root –
subdirectory1 site Root
site1
site2

subdirectory2 site Root
site1
site2

what i need to do is share the user access to both sites (sites 1&2) for example in subdirectory1 rather than have 2 seperate tables for the same product/different sites.

So effectively the url could be http://www.abc.com/sub1/site1 and http://www.abc.com/sub1/site2

Is this possible? And if not what would you suggest?

i hope that makes sense?

I look forward to hearing your comments.
Many thanks
Jocky c