Try this:
1. Add the parameters to the main report.
2. For each subreport, right-click and select "Change Subreport Links".
3. In the top left of the link screen, select the parameter from the main report and move it to "Fields to Link To".
4. In the bottom right of the link screen, uncheck "Select data in subreport based on field".
5. In the bottom left of the link screen, drop-down the list of subreport objects and select the parameter that you want to link to.
6. Save and repeat for the other subreport.
This will have the main report supply the values for the subreport parameters and should cause the parameter screen to only show them once.
-Dell