Dual 486 requires some crazy hardware hacks and a tuned OS to work. You never saw them in the wild. They were stupid expensive and required crazy hacks to make work.

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I was doing some reading last night and The Compaq SystemPro, which is considered the first server class PC, had dual 386/486 CPUs. In fact you could mix them, 486 and a 386.

It used Asymmetric Multiprocessing Processing. Basically the second CPU was an offload engine, there wasn't much OS support for it.

It was expensive, started at $16K in 1989 dollars, that is about $30K in todays dollars. So I can't imagine they sold many of them.