multiple wordpress installs w/shared user database but separate content database

Is it possible to have two wordpress installs, one at the main address and one at a subdomain, with a shared user database but totally different/separate content databases?

For example: http://www.maindomain.com and subdomain.maindomain.com

I am trying to build a community website that will allow for admins only to make posts on the main domain but allow for a shared set of users to make posts on the subdomain. I want the content to be totally separate for each install (so content from both databases doesn’t show up in a search on either site) but make it easy for users to jump back and forth between the two sites and comment on posts/message one another…etc.

Any help would be greatly appreciated!