Sounds to me like the "primary" horn depends on one of the terminals for a ground while the "secondary" just depends on the frame of that horn being attached to a good ground.
I've never heard of the horns referred to as "primary" or "secondary". One is usually a high tone while the other is a low tone. Together they sound like a typical 60s GM horn.
Follow both wires on the "primary" horn and you will probably find that one goes to ground nearby. If so, the "secondary" horn's one wire should be connected to the other (non-ground) wire on the "primary" horn. Just for giggles, disconnect the ground wire on the "primary" horn and see if it still works. If it does, the ground wire is kind of a "belt and suspenders" arrangement to guarantee the "primary" horn has a good ground.
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My Pontiac is a '57 GMC with its original 347" Pontiac V8 and dual-range Hydra-Matic.
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