[HKz]

Both arms play a large role in hitting a 2 hander, however, your body wind up and uncoiling along with the weight transfer is what will provide the most power.

The left hand is especially crucial when muscling balls especially defensively and high balls. The right hand does arguably have the least importance, as it is essentially the anchor for the shot anyways, but much of your racket head speed still relies from the right hand.

Either way, kind of a rough question to ask as some grips definitely change how you hit the ball dramatically. I know some 2 hand backhand players, who barely put their off-hand into use except as a guide while there are definitely others who muscle that ball all day with their off hand. The way I see it with other similar scenarios, whatever feels most right and proves successful just do it, I wouldn't worry too much about these details.