Hi!
I'd like to control 2 servos from my Arduino.
I need the servos to be accurate and very quick. I want one servo mounted onto the other servo, so one controls up / down, the other left / right.
I know nothing about servos, other than the very basics. Can anyone recommend a suitable servo for my purpose?
Thanks 
You might want to check out Lynxmotion's Pan/tilt kit. It looks like it does exactly what you need it to in a package that will work. Of course, you could just get any cheap hobby servos for around $10 each and put it together yourself.
Greetings,
Download one of the servo libraries from Arduino Playground - InterfacingWithHardware. I have personally used the servo library to control multiple servos, but if you are very concerned about accuracy and are only using 2 servos, one of the others may work better for you.
Good luck, and don't hesitate to ask more questions here, everybody here started from scratch some time or another.
Regards,
David