How far do your 2 intake tubes extend into the smokebox?
An inexpensive experiment would be to get some sheet metal chimney pipe that is able to either fit over the outside diameter of your forward intake tube--or fit tightly inserted into the inside diameter of the tube. Run this sheet metal chimney the length of your firebox and cap off the end. Figure out the surface of the intake and cut 4 or five smaller holes in the side of the chimney metal that together equal the surface area of the original intake. This equal amount of air opening will cause both intakes to flow evenly, and the small holes will delay entry of the smoke into the smoking chamber and create more even heat throughout the chamber. If this experiment works, then you could hard plumb it the same way.