i have XvT installed for both steam and GoG, and in the process of trying to get 3d hardware acceleration working for XvT, i noticed a rather bizarre behavior.

launching XvT from within GoG Galaxy or the XWINGTIE.exe executable in the GoG folder, will cause the steam version to launch instead, from the steam directory.

if i say, delete XvT(steam), trying to launch XvT(gog) tosses up an error that reads "unable to find xwing vs tie fighter on your system. you need to install xwing vs tie fighter before you can install balance of power." verify/repair from galaxy doesnt fix it, i have to uninstall then reinstall from galaxy to get it sorted out, but that lasts only as long as until i restore the steam version

If you installed the Steam version after the gog version maybe it updated some registry entries with the new location or something.

What if you install the Steam version and then install the gog version after that rather than restore the Steam version after gog?

Why do you need both versions installed? You can copy the whole gog game folder elsewhere if you want to mod one.

Well, I'm not sure if the Galaxy version actually does something to mess with that option, I usually use the backup.

As the user above hinted, it's very likely a registry conflict... those old titles still relied on registry keys, plus there are flags set in the compatibility database on newer OS that are set, I'm pretty sure, by both Steam and GOG installation process

In short, they were never meant to support multiple installations on a single machine... uninstall both version and pick your favorite one to use, there's really no benefit in having them both at the same time