I went from full frame to m4/3 in the last several months.
Contrary to Sony FF fans, the only way to actually save weight and size on your gear is to go to a smaller sensor (compare an A7(anything) and a 70-200 F2.8 to Canon/Nikon with their lenses. The difference in volume/mass is negligible.
Now, Take The Oly EM1 MII or the GH5: You get a (slightly) smaller camera and a smaller sensor. This allows smaller lenses. It isn't all win. The 35-80 or 35-70 are both F2.8 (F5.6 for DoF), but they are notably smaller. How much do you use the wider aperture for DoF?
The Panny 42.5mm F1.2 is a really nice lens, and I love that you can use the aperture ring if you want to (like the Fuji XT series).
In comparison to my D750 - There is less battery life, but not cripplingly so. If I carry an extra, I am doing fine. As an aside, I tended to shoot the D750 a little under according to the camera, then pulled the shadows up in post. I tried shooting the GH5 using zebras similarly, and it didn't work so well. The zebras, even at 105%, are a little pessimistic. Shoot closer to the middle of exposure, at least with GH5.
As a still camera, the EM1 is better by most accounts, but I wanted something that could double as video.
The Dynamic range falls off a little at high ISO (>3200) compared to comparable full frame, but it wasn't the end of the world once I adapted to the new camera.
I REALLY like the live preview that shows what the sensor will see.
I like having access to ridiculous shutter speeds so that I can use the aperture I want, even in sunlight.
I like the wide lens selection (~90 lenses), and competition from the 2 manufacturers keeps prices closer to reasonable.
While the lighting side is a little less fertile, you can get the really good Godox line for M43, including wireless triggers (though the best wireless setup seems to still be for Canon regardless of who makes the speedlight, this is due to camera design).
The Fuji XT line is an ergonomic dream if you like old school cameras. They are APSC, so the lenses are smaller than FF but larger than M43. They have a more limited selection of lenses and lights, and higher price tags in many cases. There also seems to be less 3rd party support.
Several pros have gotten fantastic results (see Gavin Hoey, Sean Archer, and others) using M43. There are still some nice advantages to larger sensors, but that doesn't mean you can't get great results with smaller ones.
For me, the GH5 replaced a D750 AND a Camcorder, so I saved quite a bit of space and weight. Then I have smaller lenses, and can use smaller tripods/mounts on top of this.
Having said all of this, wouldn't it make more sense if the second camera shared lenses with the main? If the goal is to take up as little space/weight as possible, that would seem to make the most sense.