Note: It doesn't matter to this question if the exercise is done lying , or sitting, it's the same issue for both.
I'm interested in the following problem: When gripping the dumbbell my hands are necessarily in a asymmetrical position, there is no way to grip symmetrically as far as I know (you can provide me counterexamples). So the problem is, the force done by each arm and hand could be really different, how to minimize this difference? Grip very strong with my hands? Alternate hand task left and right every new series?
Another option is use a barbell in which then I can position my hands symmetrically, and thus the problem is gone, or to use two dumbbells, one for each hand.
Note 2: This guy(this is a timestamp, you need to see 10 seconds of the video only) has a dumbbell with weights with large diameter, so he can position his hand in hand triangle, which seems a good grip, do you agree?