Theoritically, you can't have two instances with the same name.

Step 1: Uninstall all the SQL Server instances, if got installed.

Step 2: Install Developer Edition as the default instance.

Step 3: Install the MSDERelA as the Named instance

Use this command to install.

setup INSTANCENAME="InstanceName" SECURITYMODE=SQL SAPWD="AStrongSAPwd"

Step 4: After installation, please check the services there are two SQLServer instances running.

Step 5:Check the connection string of your asp page, to which instance it is pointing.

Step 6: If you get the same error, Open the Enterprise Manager and register your SQL Server. Go to the Security Tab and make sure, the server accepts the mixed mode authentication. Attached the gif file for your consideration.

Thanks,

Ganesh