So with Mixer it always worked, because you could release the controls so that a viewer could play. Since there's no longer a mixer, I wonder if that works with Twitch, because I always found this function really great.
No, but as I read it is this one
Co-streaming
Mixer has an innovative co-streaming feature, thanks to which up to four broadcasters can transmit their streams simultaneously on a single channel. So you can experience one and the same game from four perspectives at the same time on the Spitscreen.
really brilliant as an idea.