I can only help with Oracle 10g.
I join multiple tables from different Oracle schemas with no problem like this:
- Test your query from within Oracle
- Connect to Oracle using the same schema that you tested your query
- Paste the SQL code into the Custom SQL text box and remove the trailing ";" if you have one.
just as what jason said,
you can first test the multiple schema inside the SQL Server,
after that, you can copy and paste it to the Custom SQL.
I think there will be no problem about it.