The fuel filter on the pressure line is to "equalize" the flow of pressure to the tank, in turn causing less air bubbles in the fuel tank.

As for the dual tank mod... If your small tank is not filling completely, this may be the reason it empties faster than the large tank can fill it. Also, your engine may be running too rich. this will cause less pressure in the fuel line, and quicker fuel consumption from the 2nd tank.