We really don't recommend using it for that purpose; it's just not built for dual setup in the first place.
But if you really want to try it out, it's also quite easy to set it up.
1️⃣Plug the mixer into the gaming PC through USB.
2️⃣Next, if your streaming computer is a PC, you can use the included audio cable to connect the line-out of SC3 to the mic jack of your PC.
3️⃣If it's a gaming laptop, then you still need to find a TRS to TRRS 3.5mm audio cable. And that's all.
But the problem is that when you're connecting your streaming PC with a line-out jack, you can't hear sound back from your streaming, like the Twitch notification or something.
If you're connecting the mixer with a streaming PC through USB, you can send and hear audio back from it, but the problem is that you can't chat with your friends on the gaming PC or chat with them, and they hear the echo sound of the game.