SQL Server :: Connect To Two Databases?

I've had a site up and running for quite some time with "ASPNETDB.MDF" as database. I recently got a request to make a second site with it's own database, but that also includes a admin interface to manage the first site's database.

"Fine", I thought. "I'll just add a new connection string and point to the first database.". But unfortunately it seems to involve a few more steps then that.

My connection strings are:

<add name="ASPNETDBConnectionString" connectionString="Data Source=.SQLEXPRESS;AttachDbFilename=|DataDirectory|ASPNETDB.MDF;Integrated Security=True;User Instance=True" providerName="System.Data.SqlClient"/>
<add name="myfirstsite_ASPNETDBConnectionString" connectionString="Database=myfirstsite_aspnetdb;Data Source=.SQLEXPRESS;AttachDbFilename=C:\Inetpub\myfirstsite\App_Data\ASPNETDB.MDF;Integrated Security=True;User Instance=True" pro

But whenever I start up my first website I can't access the database through my second one. I just keep receiving the error: "Cannot open user default database. Login failed. Login failed for user 'MYCOMPUTERWEB01ASPNET'."

And if I were to start up my second one first I can do what I want, but then the first site would get the same error. So I guess the error is somehow related to the database already being in use.