It's what you always wanted and never realized you wanted! New to the signature list, courtesy of Pocket:

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Split-Effect: Average of any two effect costs. [Applies a single instance of one of two chosen effects that are both registered with the signature attack.  Each effect has an equal chance of being applied and only one of the two effects is applied per instance of Split-Effect.  The chance of either effect being applied is unaffected by RP or any other factors.  The cost of the individual effects may not exceed your signature caps.]

It's already implemented! Just check core sig attributes on the Signature Attack Effect List!