In the past when I've had a two cam shoot, usually a talking head, I either bring in an extra hand or I hire in an A7S2 or FS7 when need be.
I have never in the past liked doing this as there have been too many issues to go wrong - specifically focus when the subject moves their chair and your DOF is really narrow the whole shot can be ruined.
Now with the FX9 and the new a7sIII those problems are now a thing of the past, the greatly improved autofocus with eye tracking means you can pretty much set up the cameras and they just work.
I hated the A7SII - I mean really hated it - the menu, the form, everything was just horrible - this new A7SIII on the other hand is a revelation - much chunkier form and feel - a menu that is intuitive and easy to understand and the FN menu is inspired.
Of course when you've shot an entire feature doc on a real camera where you've learned every button inside and out these smaller SLR style bodies are never going to be the same (the last camera of this type I had was the GH4 back in 2015) it's still amazing that this little thing can shoot 10 bit 4:2:2 and slow motion in 4k at 120FPS.
For most people this will be their A Cam and I can see why, it's pretty amazing especially when paired with one of the Sigma Art lenses. Looking forward to seeing what magic comes out of this when it's mounted on the new Ronin RS2.