If the story of the five 125 gallon tanks linked up into one super tank is true I would like to see a photo of it. I do think that linking two 125 gallon tanks into one big one would work out but I think if someone had the money to get two 300 gallon tanks up like this it might be cheaper to go buy a 600 gallon tank bran new or used for the price of two 300 gallon tanks.
Here is a Link to the Square Cube Law
http://en.wikipedia.org/wiki/Square-cube_law If you double something's size it's eight goes up by eight times.