How to add duty cycle on a PWM signal
I think this circuit should be able to do the task



Vi should be the fan 1 signal and Vo the fan 2 signal. The low going edge of Vo is always that of Vi delayed by a constant time value which depends on R, C and the threshold voltages of the Schmitt trigger not gates (e.g. CD40106). 

If my math is not failing the delayed time ("d" in the picture) should be d=RC*ln(Vh/(Vh-Vlth)) where Vh is the high output voltage value and Vlth is the low-to-high voltage threshold.