You can't obivously run the *same* scenery in FS9 and FSX.
If the question is related to FSDT sceneries, we have two different versions available with their own installer so, you need to install the FS9 version in FS9 and the FSX version in FSX.
Regarding activation, the two versions share the same Serial Number and activation. If you install the two versions on the same computer, they will run under the same activation.
If you want to run the FS9 version on a different computer than the FSX version, they will take 1 activation each, of the 6 you have available, before having to contact us for a reactivation.