Ok all I know about the Z18 and CA20 engines is they both have 8 spark plugs and fire on compression and exhaust strokes.
I dont know off hand what voltage a GT40 R coil is designed to run off.
If your engine has electronic ignition it requires a 12v coil,
If your engine runs points you need a coil that runs on 7v and requires a ballast resistor.
Since your engine has 2 coils you naturally require 2 coils, both the same and for the correct type of ignition setup.
If a CA20 uses electronic ignition and so does your Z18, then the coils from the CA20 will be fine, same goes if both engines are points operated.


By the way I am not the only Auto Electrician to frequent this site, I think there is about 5 or 6 of us. I am always happy to answer questions if my brain can think of something remotely on topic to reply with.
Cheers Steve.