as long as the frequencies are the same (433 0r 434)
The frequency 433/434Mhz relates to the frequency of the remote key fob - this is what locks/unlocks the car doors and can be reprogrammed so long as you have an original key. This doesn't have anything to do with starting the engine though.
The transponder (glass tube inside the key fob) is a unique signal. But if you have the SKC code for your car (using the above ebay tool) you can allocate new transponder signals to you ECU (up to a maximum of 6 keys or something iirc) this means you can have your original key with transponder signal "x" and a second key with transponder signal "y" both able to un-immobilise your engine.
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P.S. This is from what I've read on here and various other forums, but it seems to have a variable success rate!
I'm hoping to buy both a brand new transponder and a remote key fob in the new year and give this a shot on my
mk4 Ibiza making a complete new key! I'll be try to document it and make a how-to at the time.