Beg to differ with some of the responses above.
I have an expense sharing agreement with another doctor. We share staff, office space, and some expenses but the practices are otherwise independent with separate Tax IDs etc.
We both use the same installation of AC. Each of us have our own tax ID number in AC and we can generate separate reports for things like meaningful use. The staff at AC suggested this arrangement and it was very simple to add my "office associate" as a new user to AC when she joined me a few months ago. AC sorts things like prescription refills by provider which is assigned under the demographic tab.
The one downside is the Letterhead in AC. It now says "Internal Medicine" at the top instead of either of our names. That's a little lame but not intolerable.
I think the HIPPA concern is a non-issue. Our current setup is no different than two doctors who share office space being able to go over and grab another paper chart off the shelf and read it. Or picking up a chart in the hospital for a patient who is not your own. You just don't do it.
