Since the switch transistors already are in place and working, the most simple method is to put a resistor in parallel with each transistor switch. When the transistor is off, the resistor limits the current through the fan to a value that guarantees starting but does not allow full speed. When the transistor comes on it shorts out the resistor and you have full power to the fan.
Read the label on each fan. Divide the voltage (12) by the current (1.35 A). In this case the result is 8.9. That is the equivalent resistance of the fan at full speed, in ohms. Placing an external resistor of the same value in series with the fan will put approximately half the 12 V across the fan, for approx. half speed. In fact, a fan at half voltage draws more than half it's normal current, so whenever you are rounding things, round down. Buy that resistor or the next smaller standard (8.2, 7.5, etc.) value in at least 5 W. This is a starting point; there are lotsa fan variations. If low speed is too low or the fan does not start reliably, decrease the resistor value.
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