From the Fermi theory of β-disintegration the probability of simultaneous emission of two electrons (and two neutrinos) has been calculated. The result is that this process occurs sufficiently rarely to allow a half-life of over 10¹⁷ years for a nucleus, even if its isobar of atomic number different by 2 were more stable by 20 times the electron mass.
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M. Goeppert-Mayer (1935) studied this question.