It's just basic electrical theory. To do this, you'll need two identical fans, since resistance for different fans are not the same. You can figure out the resistance by looking at the current or wattage of the fan. For example, if the fan spec is 15 watts (that would be some crazy high flow fan), you would find the current with P=VI, or Power equals Voltage (12V in this case) times Current:
So 15watts would equal 1.25 amps. Now we have the current, we'd use V=IR, or Voltage equals Current (1.24 amps) times Resistance:
So this 15 watt fan would be 9.6 Ohms. If I hook this in series with say, this 120mm fan that runs on 0.3A (40 Ohms), the two fan resistances are quite imbalanced. Kirchhoff's circuit laws come into play, and you'd see that the large 15 watt fan takes about 80% of the voltage (so to speak), and would run at ~9.7V while the 120mm fan would only get ~2.3V.
So, unless you use two identical or very similar resistance fans, the voltage would be unbalanced. Now, if you already have two fans in mind, what you'll need to do is to remove the pins from the molex connector on both fans, and hook them up so instead of being in parallel, they would be hooked up in series.
And um...long story short, I guess if you don't know what you're doing, I'd recommend you stick with fan speed 'adaptors' that use simple resistors such as these
, or fan speed controllers, such as these