For what it's worth, I put 18 gallons on a turbo 2-liter. The tank had a spill baffle in the middle, with hot return blown into the forward chamber, allowing overflow into the return chamber (back half). Outlet was from the return half, which reduced the hot water mixing from flow velocity. No problems on the long course. You might do the same effect by using two smaller tanks, plumbed together....might even be easier to arrange in limited spaces. Also easier to tell when your mixing temp is coming back too hot. I ran heavy gauge metal and 14lb cap. It'll still bend the tank when you overboost and torch out the head gasket.
I think if I was running series tanks, I'd try to push inlet flow past the outlet flow, to circulate the first tank a little (tubes passing each other internally) to prevent a straight pass hot flow into the second tank. It might buy you a little more time.
Hope this helps.
JimL