The right side holds more than left as it is not reduced by the front carb cutaway. A weir tube on the left tap can be used to increase the depth of reserve but will cause the fuel in below the tunnel height on the right to be inaccessible to either tap. When the main (left) runs out you go on to reserve (right). When that runs out laying the bike over to the right, or in extreme situations whipping the tank off to lay it on its right side gets the final few miles of fuel to the reserve tap. I will concede that flat out running on a twin may require both taps but fitting a big bore main tap to preserve the reserve would be worthwhile for road riding.