you should install them outside of both simulators. The program will make the links. Important ist just, that every FSDT addon you intend to install is in the same folder, but you will have less troubles if that is outside of either FSX and Prepar. I am not sure about how to bring it into both sims, but I think you just have to rund the installer twice and select one after the other. I am sure Umberto ore some more experienced users can help you out here.