look at
https://darkmx.app/run_mode
Put the darkmx executable into it's own folder with darkmx_local_instance_check.txt (empty text file) and when it starts up it will be single-instance but only from that specific folder. Make another folder the same way, and it can run in a separate instance. The config files will be stored in the folder with the executable, so your two instances will have completely separate configs, user name, etc.