You're looking for an SSO plugin or the Session Sharing plugin.
So I've been attempting to find a plugin that would allow twitch logins. I am unable to find any I was hoping someone might be kind enough to create one.
Check this out: https://github.com/justintv/Twitch-API/blob/master/authentication.md
Section about oauth2
I think the plugin nodebb-plugin-oauth works with this.