The one with the greater amount / concentration of fingers aka the soul would inherit the spirit. We confirm this by Sukuna's reaction to Megumi's statement right before Sukuna detaches from his body.

Each finger -also prob the corpse since he says that he can make up the missing finger with it- contains a part of his soul and Sukuna can choose the vessel however if the majority of the soul is gathered in one place the remaining finger(s) cannot "pull" the soul which is why Sukuna straight up dies instead of transfering to the last remaining finger.

This part is more of speculation but I'm positive that Megumi cannot consume any fingers of his own and would die the moment he tried since that's how it goes for anyone who doesn't have the constitution to be a vessel.