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SageWanderer Vanishingly small.
A rough way to think about it:
There are maybe dozens to a few hundred active serial killers in a large country over a long period, out of hundreds of millions of people. Even if we use a very generous estimate—say 1 in 1,000,000 people is a serial killer at some point—the chance that a randomly selected married couple consists of two serial killers would be about 1 in a trillion, assuming randomness.
But it is not fully random. Violent offenders may meet through deviant subcultures, crime scenes, prisons, unstable relationships, or mutual enabling. So the real-world chance is higher than pure random pairing, but still extraordinarily rare.
The famous example is Fred and Rosemary West, though even there the question becomes complicated: they were a murderous couple, not simply two independent serial killers who happened to marry. Another example sometimes discussed is Paul Bernardo and Karla Homolka, though classification issues arise there too.
Bottom line: as a random event, almost impossible; as a folie à deux or mutually reinforcing criminal partnership, rare but documented.