Users should never install both. There is no reason for a user to require ScriptX.Addon and ScriptX.Services to be installed.
If they are a user with Internet Explorer 11 to access applications using ScriptX then they should install ScriptX.Addon.
If they are a user with Edge, Chrome, Firefox etc. to access applications using ScriptX then they should install ScriptX.Services.
Developers may want or need to install both as they transition a site from using ScriptX.Addon only to supporting all flavours of ScriptX.
This is fine, it works. We do it all the time!
However, things will go wrong if you uninstall one of the products; the remaining product will no longer work correctly. To resolve this, repair the remaining product or uninstall and install again and all will be ok again (and the uninstalled product can be re-installed again at some point in the future without any issue).