PC Fan PWM signal circuit using 555 timers: Can anyone explain this circuit to me?
One of the 555 timers in the 556 is used as a typical PWM generator.  The second 555 is used to buffer the output according to that article.  You may only need the single 555 circuit detailed in the first part of the article.

The effect of the pot is to vary duty cycle from about 5%-95%.  The frequency will be based on the resistor, pot, and cap values, and held fairly constant (although there will be some slight frequency variation as the duty cycle changes).  The diodes types (forward drop) will affect frequency also.  This article may explain the operation more clearly.  

You could set up two 555s (or a 556) as you described to maintain frequency if needed.  I think a fan would work fine with the frequency changing over the range produced by the circuit as shown.  

There are more elegant solutions to this (a microcontroller would probably be best), but 555 is most likely the simplest solution you would fine, and probably more than sufficient for the application.