key programming

[Niall]

so if you got 2 keys from another car you could program them in and have you're old one no longer working?

No. You have to think of the key as two separate items

Immobiler. Each key has a tiny chip that has a unique ID. You can program up to 5 of these to start the car.
Central locking. Once again you can have up to 5 but it works the other way round. The car BSI has a unique code which is written to the central locking chip in your key. Once a code is written, it can't be changed. The key chips are 'one use' in that sense but there are rumours of it occasionally working.

So if you don't mind your RCL not working, programming keys can be done very easily with keys (new or old), pug planet and your security code for your car.