Two Fans on One Wall Switch?

If installed by an inexperienced do-it-yourselfer*, they may have wired the fans in SERIES, instead of in parallel as they should have been. That would make the fans divide the voltage between them, in effect giving each one half of the voltage it was designed for. Many ceiling fans are designed to operate from dimmers, so reducing the voltage to them is allowable and is how your reduce their speed. So it would not harm the fans, it would just not allow them to run at their designed maximum speed.

• This could have been done on purpose by the way, thinking that full speed was too much for the space anyway. I've seen that done at restaurants so that customers or workers cannot turn the fans up to full speed and make a mess.