A dual exhaust system may or may not increase your fuel economy. It depends on the individual vehicle.
I'm willing to bet, however, that when it's first installed, your fuel economy will
decrease because you'll be sticking your foot in it to hear the "more powerful" sound coming from the dual exhaust system.
That said,
MadisonMPG's suggestion to remove the 6" lift and return to the stock tires
will increase your fuel economy. "Adjusting the nut behind the wheel" can also make a significant difference.
Please post a fuel log for your Tundra. I would be interested in seeing the progress you make with your fuel economy. I'm sure a few other members would be interested, as well.
Mid 20's from a Tundra would be quite impressive.