With Immo-3 it was possible to have one key working with multiple clusters. The trick was to match the second cluster to the original car's ECU first. I know a guy who had three different MY 2004 cars (two TDIs and a Golf-R, all Mk.4 chassis) and the same key would work in all of them. Of course, they all thought they had the same VIN too.Quite the opposite Don, I'm afraid. The 'Cluster Code' (a series of about 7 bytes) gets written to the key when programming. Since about IMMO3 the chip in your key fob is program once only. It may be repeatedly programmed to the same cluster however, (when eradicating lost fobs for example) as the Cluster Code doesn't change.
I do not know whether this sort of trickery is still possible with current generation immobilizers.
-Uwe-