This study investigates bilectal grammatical representation and processing using three ERP reading experiments in two Norwegian dialect regions. Northern Norwegian bilectals were tested in two separate sessions in two written varieties: the local written standard (Bokmål, n = 83) and Northern Norwegian dialect writing (n = 68). The study included both non-contrastive gender (control) and dialect-specific number (target) agreement conditions. In grammatically incongruent number conditions, participants display contrasting processing profiles in both on-line and off-line measures (reversed P600 components and reversed grammaticality judgments). To further test the interaction between contrasting bilectal grammars in language processing, the Bokmål version of the experiment was also conducted in a second dialect region (Sunnmøre, n = 73) where the spoken dialect is grammatically aligned with Bokmål for both gender and number. In the Bokmål mode, compared to both the control group (Sunnmøre) and the control condition (gender agreement), Northern Norwegian participants in the target (number) condition show significantly attenuated ERPs and more gradient and less accurate grammaticality judgments, evidencing competition between distinct bilectal grammatical representations. The results further revealed significant individual differences in the degree of cross-dialectal influence between Bokmål and Northern Norwegian dialect modes, contingent on individual participants’ bilectal engagement and exposure. Together these results suggest that bilectalism is a proper sub-case of bilingualism: bilectals develop distinct grammatical representations for contrastive grammatical features in distinct L1 varieties with which they have sufficient engagement and exposure.