Furthermore, the dilution effects won't cancel exactly. Since both redox couples involve 2 electrons, it takes one mole of triiodide to titrate 1 mole of ascorbic acid. To do this by back titration, first assay the tri-iodide solution by standardizing against arsenious acid. Then add the unknown ascorbic acid to a known volume of standard tri-iodide. Ascorbic acid should give a sharp first end point.
Any analytical problem has many solutions.
Sometimes you're stuck using the tools at hand. There's usually something in some corner of the chemical literature that you don't know about. You can only choose methods from the universe of which you are aware. Courses, of necessity, do not reveal the entire universe you'll find useful. Yes, I know the answer's in the back of the book.