I think it will depend somewhat on the type of paddling you will be doing. I keep a 3L camelback behind the seat of my Nordy, and that is about all that will fit under the backback to bulkhead straps.
You are talking about nearly 30lbs of water that will slosh around big time. This will not be much fun if you are already pushing your limits in conditions. If you have spent much time paddling with a partially flooded cockpit, you know what I mean.
If you can figure out a way to secure the bladders to the bottom of the boat it would be helpful, but sloshing will always have some effect even just while just leaning the boat. If you are paddling moderately flat water, then it would be reasonable to go for it, but I would do some serious testing before I headed out on a trip expecting to do this for the first time.
Cartop Kayak Carriers
Recreational Kayak Paddle
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