programming a key (1994 xantia)

I have a 1994 xantia and 2 keys for it, but only one of the keys work with the remote. I have tried reprogramming the other key by:
ignition on 1 click point key at the IR receiver hold top button for 5 seconds hold bottom button for 5 seconds turn off ignition
This sort of worked, it made the 'new' key work, but the old key which was fine before stopped working (by IR). I guess what I did reprogrammed the car to accept codes from the new key instead of programming the new key to work with the car.
Does anyone know if it is possible to reprogram the new key so that both of them work, or is reprogramming the chip in the car all I can do?

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