The simplest way is to set
--multi-thread-streams 0
Non-backend flags can unfortunately not be set in the config (yet). NCW is aware of this and intends to improve it in future. Meanwhile you will have to set it in whatever launch script you use. I'm not aware of any other ways to set it (but that doesn't mean it doesn't exist). Usually environment variables will be listed in the docs under the spesific flag, and it is not for this one.