4n,
ive never liked a typical piggyback setup, especially using aerator pumps. 1) the flow rate slows dramatically once the hard tank is full and you start pushing water through it to the sac, which is often at a higher level. 2) you have to be careful that the hard tank is allowed to still vent freely. If not, it will tin-can on itself and can crack. This would occur after the sac has gravity drained to the tank and close off the vent thats now through the soft sac.
Having the filled sac higher than the cent or drain outlet, is certainly a recipe for passive draining. if you are going to list the boat for surfing, you can cross the vent and drain line to the other side. Do both sides.
Without knowing more about this particular ballast setup, I would consider giving each new sac, its own dedicated plumbing, leaving the original intact.
Michael
Mikes Liquid Audio