Depending on what size bags you're talking about that's gonna be some long fill times. Is it an impeller pump or aerator pump? I don't know anything about Sangers but I think I've seen people say that they have aerator pumps from the factory. So it's probably something like an 800 GPH pump which if we're talking about 400 or 750 lb bags in the rear you're talking about 8-12 minutes per bag. With more weight in the back you're likely going to need more in the front too (Unless this is just for surfing). So, you're going to end up with 30+ minutes worth of ballast filling if you're filling them in series like you're suggesting. I.E. Fill the front for 5 mins, allow it to overflow into a bow sac for another 5+ mins, flip the valve, fill the port rear for 10 mins, flip the valve and fill the starboard rear for 10 more minutes.
Also, if it is an aerator pump that you're referring to it's only good for filling since it's not reversible. Your tank likely has another aerator pump connected directly to it for the drain. If that's the case you're going to need drain pumps connected to each of the new bags too.
In short I think what you're suggesting is just going to result in a convoluted mess of hoses that won't meet your expectations. You need to get those tanks/bags filling in parallel with at least an 800 GPH pump for each tank/bag to get reasonable fill/drain times. Join the club and pop some new holes in that hull son! Measure 10 times and drill once. It's not that painful after the first one's done. You kinda need separate vent thru hulls too so you know which bag is full. If not when water starts coming out of the vent you won't know which one is full.