you would not want the pump out line to be a T between the tanks. it could only empty one side, if the other has the slightest blockage.
honestly if you can run a line between the 2 to have it use both tanks, you could just run a new line from the head to the new tank, the longer line wont really matter, same with the vent and pumpout point. remove the old tank and run new longer lines.
i really think you are asking for problems if you try 2 tanks. its just 2 2 inch lines to extend, if you tried an interconnect line it would need to be at least 3 inch just to get "stuff" to flow between the tanks