Great idea. Might as well set it up for the best of both worlds.
If you want to run it as a single, your best bet would be to just set it up as a single. Get a two into one adapter up front, 2.5in single pipe to one muffler, then either one 2.5 out the back or if you want to keep the dual look split into two 2in over the axle and out the back. The single 2.5 out the back would be lighter. You can put one muffler back in the garage for when the other wears out. Going single line would likely drop about 30lbs off the car, and a proper single line might get you to that 35MPG goal once the computer's had a chance to get used to it.