You shouldn't have a problem calling different reports at once. As long as you're using Forms 11g they show up in different windows.
The question is how you are calling the report.
Here is how I manipulate it.
After I pass parameters with the ADD_PARAMETER built-in I set some key values (destype, desformat, desname) with the RP2RRO's procedures.
Finally calling RP2RRO.RP2RRO_RUN_PRODUCT and then WEB.SHOW_DOCUMENT passing the correct procedure parameters the report comes up in a window. If you repeat the above changing the appropriate variables (the report_name in the RP2RRO_RUN_PRODUCT and so on) you can get multiple reports in different windows.
When i try to do that, the report always comes up in the same window.