I thought about two containers vs one large one. Rarely would I ever need the 4 gallons that the large one holds, but it seemed better for a couple of reasons.
First, the weight of the large one is less than two smaller ones. And you still have to add in the weight of two mounts.
Second, the expense is way higher for two Rotopax and two mounts than one single.
Lastly, if I have run my gas tank down close to empty, I really only want to deal with filing the tank back up one time.
I agree that it is large and convivence of the smaller Rotopax would be nice. For me, I made the compromise to get the large one. And it fits really well against the ROPS on the 520s.