Here is a little theoretical breakdown... (Yay math)
For the sake of example, let's say you change exactly 50% of your tank water every Sunday. Assume at the end of your average week you have 20ppm nitrate before your 50% change. After that change you would have 10ppm. If on Wednesday (halfway between your weekly water changes) you had 15ppm (halfway back to the end of week total of 20 from the 10 you started the new week with) and you did another 25% water change as an extra you would have 11.75ppm after that change. By the next Sunday, assuming your tank generates 10ppm a week (end of week total is 20, taken down to 10 by the 50% change and then goes back up to 20) your tank would have 16.75ppm of nitrate. The 50% Sunday change would put you to 8.75 to start the new week. Using the same schedule, by the end of the next week you would be at 15.0275ppm, by the next week at 14.386, and by 5 weeks of this schedule at 14.01ppm. It would theoretically keep dropping a little more each week, although the difference will essentially even out to around 14ppm instead of the usual 20, assuming waste production is stable.
Now lets assume a 50% change twice a week on Wednesday and Sunday with the same weekly waste production. On Sunday you start with 10ppm after your 50% change from 20ppm, and on Wednesday you have 15ppm again, but you decide to do another 50% change, by the end of the week you would have 12.5ppm. Then you do your Sunday change and start the week off with 6.25. Another week of this and you would end the week at 10.62, the next at 10.15ppm, similarly staying around 10ppm at the end of the week.
Essentially, as you would probably guess, the amount of this second water change would reduce your end of week total nitrates by the percentage that you change. I just felt like playing with numbers for a while to give you a better picture of how adding a second water change might affect your overall water chemistry. Also remember that I totally pulled those numbers out of thin air, as I am not sure what your nitrates look like after a week between water changes. Just used 20 because it is the generally accepted "safe" level by many people, and of course the exact daily rate of production will probably vary from day to day and week to week.
And I think that is the most math I have done at one time since my last math class...