Several years ago I tried to make a fan controller that would allow two fans to match RPM's. The goal was to avoid having two fans at a slightly different speed, which would cause an audible effect called beating. This effect is commonly heard with dual-engine turboprop planes. When the fans/engines are close in RPM you hear a whah-whah-whah sound at the difference in RPS (revs-per-second). For example, if one fan is spinning at 1000 RPS while the other fan is at 1001 RPS then you will hear a whah-whah-whah sound at 1 Hz.
My circuit had 2 fans, both with tachometer outputs. One fan was controlled with something similar to what you have now. The second fan was slaved to the first. This was done by using a circuit very similar to a Phase-Locked-Loop (PLL). Here's a simplified diagram of a PLL:
Ref Clock is the tachometer signal from the first fan. The Voltage Controlled Oscillator (VCO) is actually the second fan, with the tach signal going to the divider. In your case, the divider can be bypassed (divide by 1). You'll have to do some research into Phase-Frequency-Detectors and Loop Filters, but you could probably use a simple XOR gate as the PFD.
The basic idea is this: The output of the loop filter IS the voltage used to drive the fan. Because the fan speeds up as the power voltage goes higher, it works as a voltage controlled oscillator. If you get the PFD and loop filter correct then the second fan will be the same speed as the first fan.
My circuit mostly worked, but I ran out of time to tweak it to work perfectly. The two fans were very close in speed, but not perfectly matched. The fans I was using would have some speed variations that the circuit couldn't adapt to quick enough. I wish I had time to finish it, but the early results were promising.