This has been asked and answered so many times on the forum, and it's also indicated on each product page, for example KLAX:
http://www.fsdreamteam.com/products_klax.htmlThe price include license to use both the FS9, FSX and the Prepar3D version.
Which means the same Serial will activate both, because you are not buying an "FSX KLAX" or an "FS9 KLAX", the only product on sale is "KLAX", so you'll be getting one purchase receipt with one Serial number that will be valid for all supported simulators.
The activation is entirely unrelated to the sim you use. You activate KLAX on *Windows*, not on FS9 or FSX or P3D. And, it's entirely unrelated to the sim installation status. You can uninstall/reinstall the scenery or the simulator it runs under an unlimited number of times, but it won't affect the activation. The ONLY time you need to reactivate, is when you reinstall Windows or you change hardware components.
Which means, if you are running FS9 and FSX under the same Windows installation, if you activate the product under FS9 first, it will be already Active in FSX without doing anything else.
If you are running FS9 and FSX under different Windows installations, either because you are dual-booting between two different boot drives, or because you are running FS9 on a PC and FSX on another one, you will have to activate each product under its own Windows install separately.
Of course, you will still have to download and install the correct version for the sim you use, so the FS9 installer under FS9, and the FSX installer under FSX, P3D or FSX:SE.
And note that, regardless of the license policy, there are products that are ONLY available for FSX/P3D, like CYVR, KIAH, KJFK V2 and GSX, so there's no way to use them under FS9.
