Hopefully you folks can clarify some of this for me. I have a web-application using the Sql Membership Provider and it talks to a second web-application through a WCF Servi
I'm assuming your WCF membership provider and your web application membership provider are using the same backend user set.
If so you would want to share authentication cookies between the applications. You can find more information on how to do that here.
Next you need to pass the authentication cookie that came with the web application request to the WCF service call. You can see how to do that here.
The idea is the user logs into your web application and by doing so is logged into your WCF service.