Reports Manager is not designed to run against multiple databases on a single run. If script output will suffice, rather than a report, Script Manager can be used:
Go to Utilities | Script Manager
1. Click the Add button. Here you can multi-select the connections you want to use
2. Click on the 'New DataFile' button to create an .sdf file to store your settings
3. Click the 'Add script' button to add a .sql file that holds your specific query
4. Click on the 'Options' button to select 'Spool Output to file' options (no formatting is available)
5. Click the 'Schedule' button if required.