I did the same. I covered one to see if I noticed any sound improvements and did not notice any changes in sound quality. Having the dual tweeters seems to allow the power to be spread out when I crank the volume as to not overload a single tweeter. At least that's how I perceive the reasoning behind the second tweeter in Axiom's design.
People don't realize just how high a note tweeters play. Take a look at the crossover points (e.g. 2Khz, 3.5 kHz) and then play a test tone at the BOTTOM range of the tweeter. A 3kHz note is pretty damn high. People are far more likely to hear differences in the midrange when it comes to overall quality.
Try covering your second midrange driver and see what happens. If it cuts the midrange output (as expected), again, you will likely notice more of a difference there than with the higher tweeter range frequencies.
