Whiel I understand wanting to use a variable name for this, I think you are going to be better off doing it more like this:

<connectionStrings>
 
  <add name="DB1_ConnectionString" connectionString="Data Source=serverName;Initial Catalog=DB1;Persist Security Info=True;User ID=pepe;Password=****" providerName="System.Data.SqlClient" />
 
 
<!--
Comment out the one you don't want

<add name="DB1_ConnectionString" connectionString="Data Source=serverName;Initial Catalog=DB2;Persist Security Info=True;User ID=pepe;Password=****" providerName="System.Data.SqlClient" />

-->
 
</connectionStrings>