What a (healthy) dual has to offer you is unmatched by any other type. It seems semi-duals find each other faster than duals because of certain commonalities, but that doesn’t mean they’re more compatible.

In a semi-dual pair, each is forced to take care of some things by themselves. It fosters independence - which can be a valuable trait, but not my ideal. Beyond the initial “oooh, shiny!” attractiveness, the intruding semi-dual in the OP’s question would be hard-pressed to try to separate a dual pair that has gotten used to one another.