I have been trying to setup authentication between SharePoint and another asp website. SharePoint has been setup to use FBA membership provider, the same database used by the asp.net website. The problem is how to login to one of them, I would prefer it to be sharepoint and then be able to get to a page in the other without logging in again.

I have tried setting this link, but using sql membership provider, but I don't think this could work as the way auth cookies are used by browsers.

You can't share auth cookies between site1.something.com and site2.something.com, only something.com/site1 and something.com/site2 as I understand it.

I would be happy either way but would prefer to keep them separate in IIS.

Does anyone know if this is possible.

As you discovered, pointing two apps to the same SqlMembershipProvider just means that they share logons.

what you are asking about is Federated Authentication... whereby an external system provides the authentication to one or more (web) applications... SharePoint calls this Claims Based Authentication... open specs include OpenID, OAuth/OAuth2...

both apps will need to be converted to support them (and their use is DRASTICALLY different than the asp.net membership provider, so use dev/test systems before you roll it to prod).

But basically, when you access the webapp, it checks for its own login cookie... when it fails (first time accessing the webapp, or login expiration), it redirects user to external auth system, which has its own cookie as well... user auths, auth system stores its own cookie, redirects user back to webapp... user then goes to other webapp, again it checks for its own login cookie, fails, and redirects to external auth system... since external auth system still has a good cookie, it redirects user back to (second) webapp automatically... user never had to type creds a second time.