There are a couple of ways they might do it - and I'll be general here and this may not be a thorough list, as I don't actually do this type of content myself:
- Their face camera has its own media so it's doing its own recording (like a mirrorless or DSLR or something)
- They use two different recording softwares or capture devices to capture the two separate angles
- They record with one screen recording application, but they record twice the width, so if you had a 1920x1080 screen recording, and a 1920x1080 camera recording, you would have a "canvas size" of 3840x1080 where they'd have the face cam on one side and the gameplay on the other side. Then they would just put that very wide video into a 1920x1080 sequence and move it left for one angle, and right for the other angle.
With all of these scenarios you'll have to keep in mind that you either need the hardware or the computing power to be able to do these recordings while also doing whatever you are screen capturing. Some of the workflows that would involve a camera or an external capture device could help offload some of that processing so that the computer isn't the thing doing all of the work.
Thanks Phillip, I say the 3840X1080 technique on youtube and that works great when recording content, but it doens't work when streaming as your display screen has been adjusted. I've ended up using a second webcam on recording on a seperate bit of software on OBS so I can record myself in full screen at 1080p whilst I'm streaming. Just suprised me that there was not an intuitive way to do this on OBS