Right now OpenMPT can only be put into shared instance mode by adding the /sharedcommand line parameter; this can be difficult if the file association was made by hand (e.g. for a file extension not officially used by OpenMPT). It might be a good idea to have a (hidden, for now) setting in the INI file which would also enforce this mode no matter how file extensions were set up. Obviously this is not quite as ideal as the command line parameter because it cannot be caught as early (and will thus be a bit slower), but may still be the easier solution for some people.