My longest lens is a 50. Maybe, sometimes, a rental 85. All primes.
Could make a case that left hand could clearly support the body, and also run the joystick controller for focus point placement.
The lens mount on the camera is not designed to bear the weight of any lens more than a pound or so, roughly the weight of the camera itself. With long lenses, the lens is designed to hold the camera instead. You absolutely need to get a hand under the lens or the lens mount will bend.
If one tried to work with both thumbs behind the camera, I believe one's wrists would be in terrible joint-stress pain after a day or two. The wrist is better as a passive support, with the rest of your body directly underneath it.