I've seen several videos where there were multiple windows showing front facing or cockpit facing in conjunction with rear facing.

How is this done?

GoPro's

With editing software? and dual cameras...

I remember meeting a guy a while back, here or at the club who was working on a hardware & software package that would overlay a gps created map of the track with 3 or 4 cameras. It was way before the GoPro days. You'd hook up 3 or 4 lipstick cameras into their box then into a video recorder, you could set up a quad split, picture in picture or a few other configurations. I'll have to do some research on this, I did some vinyl work for them.

It was View-2.com, and their webpage doesn't work anymore.

https://www.youtube.com/watch?v=AjGXn249Fc0

I'm still here. I've made the boxes for years that accept up to 4
standard def cameras and also on-screen data. Accelerometers
are in the box, and GPS plugs in to do speed, co-ordinates,
altitude, and true time. (Other applications include stunt planes)

A few years back I decided to let Datatoys.com brand this and
do all the selling, and they added a foolproof cable harness for
everything as they sell cameras, etc.

I'm sure you can still get systems from them, though sales have
slowed a lot since the advent of HD, onboard encoding and SDIO
cards. But there are some things that just call out for real-time
muxing without wasting time with editing.

GoPros are cameras. You need a mixing system. What do you use Jeff?