As an update: Considering it's difficult to achieve this (no matter whether I use acceleration or force mode, I cannot use 2 different force values to apply force on 2 ropes, which may make the bouyancy simulation unreal) , I finally decide to use the acceleration value calculated by main rope's density. Fortunately I'm developing a 1st person game so the sub rope is usually far away from the camera and it may get visually unreal bouyancy only when the hook breaks, so game player may not notice this in my game.
I hope this feature (1 force zone, different force val/direction for different actors) can be supported in future version.
I hope this feature (1 force zone, different force val/direction for different actors) can be supported in future version.