As in take video from two machines (such as for a co-op game), and integrate both onto a single stream? Get a capture card. One machine will be the streaming system, responsible for capturing the video sent from the other machine, compositing, encoding, and sending the signal out.
If you mean from your machine and a remote co-op player, good luck. There is currently no good solution for this.
If you mean a 2-PC streaming setup with one system being the 'gaming' machine and the other being the 'encoding' machine (with no game being played), pick up a capture card (as above). Alternately, you can run nginx-rtmp on the second machine, and on the primary run OBS and run it with local-recording settings, pointing at the other machine which will then re-encode the signal before sending it out to Twitch/other service. This method requires a lot of technical mien.