As clearly indicated on every page of every product:
The product is sold as 2 Products in One. This means, by buying one version, you will automatically get the other for free, since activation and serial codes are shared between FSX and FS9.
You simply download the FSX installer and it will show as active already in FSX, if your FS9 install was active on the same Windows installation.
If, instead, you want to reinstall Windows from scratch, or you want to use FSX on a different PC, you just select "Register Serial" and insert your Serial Number to activate it on the new machine. This will use 2 of your 6 activations, though.