SSRS Dynamic Data Source through an Expression-based Connection String

I wanted to use single query to retrieve data from AdventureWorks2012 and AdventureWorksDW2012 in one dataset:

1) I added "AdventureWorks2012" and "AdventureWorksDW2012" to Data Sources;

2) I created two parameters--@ServerName,@DatabaseName, and added two datasets

--servername--
select srvname from sys.sysservers

--databasename--
SELECT name as DatabaseName
FROM sys.databases
where database_id in (7,8)

3) I created a "DynamicDS" data source as below:

4) When I tried to create a dataset by using the "DynamicDS", I got an error message:

How can I do with it? Please help me. Thanks.