It's not good for you to shower twice a day and it doesen't mean you are more clean. Infact it starts your skin creating acne and shit when you shower more than once a day, and i sure as hell hope you don't wash your hair twice a day too.
With that being said i usually take 1 shower a day but when when i do sports/work out i will shower after.
I think that is kind of a myth about acne that has been circulated. People have assumed that by showering once or twice a day, the skin has to produce more oil, which in turn leads to more acne. I don't know that this has ever been proven outside of some kid saying on the internet that his acne got worse when he showered more.
I think showering twice a day is fine. Soap can lead to excessively dry skin, so they may tell you to not use soap in both showers, but other than that, you aren't going to develop skin problems from showering twice a day. Of course, as with anything, some people will have problems where others don't. I shower anywhere from 1-3 times a day depending on what I've done and have never had any issues with dry skin or acne.