I tried it a few times when I was getting sick of getting dust on my sensor changing lenses when out hiking. I was shooting wildlife with one body and wanting to have another body for landscape and general purpose.
It didn't really work out, and because it's relatively clumsy to do, I never really did it after that. Main problem was that I was swapping between two different systems (Nikon D7200 and a Canon EOS M), and the shooting style and controls were throwing me off. I still think that, in concept, having a little mirrorless camera with standard zoom lens makes a lot of sense to have as a backup when shooting with a more limiting lens on your main camera, like telephoto or wide angle, but I think in practice, both bodies probably need to be basically the same model or same system. And at that point, the added weight of two full size cameras around your neck might weigh you down so much that you won't feel like staying out as long.
