Would it be less effective to only use half of a box of Crest Whitestrips?

I am currently using the 3d advanced vivid whitestrips. In every box you buy there is a $7 off coupon for another box if you are wishing to buy two. I didn't Start seeing results after three days. After about half the box (10 strips) I started to (while using the toothpaste and mouthwash). I am almost done that box and am going to start my second one right after even though they say not too. I wouldn't recommend splitting a box even though it is the highest level of whitening out of every other kind of crest whitestrips because although they do work, they don't work as well as they say.