Are the two cables interchangeable? I have a messed up return cable. I was planning on getting two new ones but don't see the reason if I can just replace the one.
Not directly. An "A" cable (open) can be replaced by a "B" (close) cable, but its not a direct swap . I do not think the A cable can be used as a B cable at all.
The A cable has an adjuster on it , the B cable does not.
If you must choose, run a used B cable as it does nothing unless called upon. If both are adjusted properly, only the A cable and the return spring do anything. The B cable is called upon if the return spring malfunctions, or the A cable hangs up.
Buy a new A cable, if yours is old especially, it will feel so much better.