You may stream to multiple platforms at once, however you lose your ability to be verified on twitch by being an affiliate/partner.
I see more and more of the large platforms taking this route and removing the verified/special perks for being on that particular platform if you are not keeping your content exclusive to them, in an attempt to keep the largest market share/content creators.
In the end I think this will only drive the more independent creators to adopt smaller and more open platforms and the big platforms that are left will be an echo chamber more than ever.