Any difference between fobs?

I bought a used key on Ebay recently and a locksmith programmed it with no problem. If you pull the key out of the fob, you can see 7 or 8 lines of numbers. I was able to find a key with all matching numbers except for the top line which I think is unique for each fob. At a minimum, the FCC number, the model number and the number of buttons needs to match.

If it were me, I would try to program all the fobs to both the cars.

RalphVa said:

Didn't do the MX-5 fob the same. They both have keys (for glove box, doors and trunk on the 3, and for ignition on the MX-5). Would have had to change the locks on all the stuff that took the key to have them the same.

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Could you have programmed the transponders the same? That way you would have only one fob and you could carry a simple flat key or hide it on the outside of the car.
FWIW I carry my fob in my wallet with one of these, so the one fob solution is way better for me:
https://mx5things.com/2017/10/28/car-key-in-the-shape-of-a-credit-card/