Is a shared session an option?
WCF offers an attribute (AspNetCompatibilityRequirements) that (presumably among other things) makes the session state ASP.net compatible. This in turn allows you to use out-of-process session state management that can be shared between applications.
You can enable this using
[AspNetCompatibilityRequirements(RequirementsMode=AspNetCompatibilityRequirementsMode.Allowed]
And then in your configuration
<system.serviceModel>
<serviceHostingEnvironment aspNetCompatibilityEnabled="true"/>
...
</system.serviceModel>
If you go the SQL Server route, I wrote this a while back. It is sharing between asp.net and an asmx service, but on the SQL Server side I imagine things are the same. Basically you need to make SQL Server recognize both parts of the system as the same application.
WCF also uses the same membership providers as ASP.net, so you might be able to get something going that way.