Thank for the idea of adding a pump that will transfer the water from one sac to the other. I will not help for the filling of the 1st sac but then for changing side it will be efficient: with 2 pumps on each sac, one will drain faster and the other will obviously fill faster.
The reversible pump feels like a natural choice: ony one pump to operate and you can also use it while riding to adjust the wake when necessary.

Do you know where I can find a switch "moomba style" to operate the pump?