The activation is just one, and it's not tied to FS version, but to your windows/hardware config.
So, 1 activation will works in both sims, you don't "consume" 2 activations if you install in FS9 and FSX.
Also, the activation is stored in Windows so, there's no need to have both FS9 and FSX installed at activation time. For example, let's say you only have FS9 installed, you can download, install and activate the FS9 version today and use it normally. If you install FSX at a later date, if you install the Trial for FSX, it will automatically run as fully registered without doing anything.