Your original question stated you would need to rekey all four doors!!! Do you have a key now for the rear doors?? If so, which car has exterior locks on the rear doors??
Why not save money and just put remote openers on both cars that use the same frequency or remote? They are much easier to use and nice to have. Then you can unlock the other doors once inside and just leave the key in the ignition or in the console.
This would save you a bunch of money on parts and installation.

Tobias MB
3 MBs