Aims and objectives: Grammatical rules in one language that induce the speaker to switch to another language (Matras’ ‘bilingual suppletion’) are reported for two languages: Beni-Snous Berber (as described in Destaing’s 1907 work Etude sur le dialecte berbère des Beni-Snous), and Jerusalem Domari (as described in Matras’s 2012 work A grammar of Domari). Few details are available, yet the two cases show greater similarities than expected if any grammatical rule could specify switching. This paper seeks to describe the phenomenon more precisely and to provide a principled explanation for the similarities. Methodology: Destaing claims that numerals greater than 10 obligatorily select for Arabic nouns, but provides few examples and no frequency indications. The description was made more precise by examining all numeral+noun phrases in published texts, supplemented by elicitation. Contact effects were distinguished from retentions through comparison with nearby Berber and Arabic varieties. Data and analysis: Destaing’s text corpus yielded 213 numeral+noun phrases; another 169 were elicited in 2013 from seven surviving speakers. In both datasets, counterexamples to Destaing’s original claim were found, but switching was restricted to numerals greater than 10, and Fisher’s exact test confirmed this factor’s significance. Findings: Beni-Snous Berber – like Domari – shows a statistically significant tendency to use Arabic nouns with numerals for which Arabic and Berber selectional requirements conflict. Modern speakers additionally show optional syntactic calquing in such cases, accompanied by fewer switches. These facts are predicted by the hypothesis that ‘bilingual suppletion’ is induced by words shared across the two languages with different selectional requirements. Originality: ‘Bilingual suppletion’ remains little-researched, and its relative frequency in corpus data has not previously been examined. No restrictive explanation for it has previously been proposed. Implications: The results indicate that ‘rules’ inducing language switching (always optional) can emerge naturally from conflicts between grammars within bilingual populations, rather than needing to be directly encoded in the grammar.