You shouldn't really have a difference of much more than +/- 1 or 2 degrees from your actual humidity if your hygrometer(s) are accurate. The difference between the two is 5 degrees. If you want to get an idea of the accuracy of the hygrometers, I would suggest doing a salt test on both of them.
Now you don't have to get rid of the one that is off, just make note of the differences and watch for fluctuations.
On a side note, what credo are you using, foam (w/ distilled water and 50/50 solution) or beads?