Computer can show different "widgets" but screen space is limited. With 2 computer I can run twice as many widgets, the screen space cumulatively is larger, thus, in a pocket you'd waste valuable access to dive info, plus, cumulative illumination can be a flashlight in itself.
It is nice to glance once and quickly read info in one place. On different hands it takes too long, specifically if you hold a tool or a flashlight, how can you activate the buttons? If you are to place the computers on one hand, during a dive they list and screen angle becomes too much, imperfect LCD technology makes a screen darker at an angle, you'd have to rotate it back, much maintenance during a dive.
Another problem is the side buttons. You cannot put shearwaters next to each other, must have space clearance for the buttons access. This creates a problem, while one computer can stay on the narrowest part of the wrist, the 2nd computer has to be closer to the elbow. During a dive it always crawls toward the most narrow part of the wrist. When it does, it becomes loose and may even fall and hang. Again, proving it is too much maintenance to keep it in place.
With all this, it is evident that anyone would want them both mounted side by side, on a rigid backing.