How to dynamically change a datasource in SSRS (MS SQL 2008)

I am looking to dynamically change a datasource in SSRS (Sql Server Reporting Services) to connect to either one of 2 Oracle databases, based on a parameter.
I use the function icon when setting up my datasource, and enter the following function:

="Data Source="&Parameters!Source&";User Id=MyId;Password=MyPassword;"

The parameter source contains the TNS Name of my Oracle database

When I run the report, I get an error "Can not create connection to datasource, null password given, logon denied"...
So SSRS sees my connection string, but for some reason is not able to parse the username and password...

Thank you

Olivier