Hi!
If the circuit works with 1 fan should work with 2. Unles pin 1 (disable) from Arduino isn't connected on pin 1 - 7 on ULN2003 as you can see on schematic.
Or your 12V power supply can't give enough current to all devices.
Regarding speed, which temperature you have configured to fans?
Images of your circuit can help but, please post images correctly.
Best regards.