1. Create queries you would like to join.
2. Open one of the queries in BEx Analyzer and save it as a workbook.
3. Create two additional tabs in the workbook, and give them names (e.g., query2, results)
4. Edit the query2 tab by adding design items: click BEx Analyzer > Design Toolbar > Insert Analysis Grid
5. Click on the Properties dialog box, change Data Provider's name and click create button.
6. Choose the second query and confirm your choice.
7. Create a table, on the result tab, that merge data form both queries. Save the workbook.
What about selection screen? The variables related to the queries will be displayed on one selection screen, if you use the same variable in the queries - there will be only one field for the shared variables.
To learn more about analysis grid, read Analysis Grid section at sap.help.com.