It is well established that early bilinguals who speak languages that differ in the phonetic implementation of the voicing contrast have language-specific voicing systems. This study investigates voicing separation in bilinguals who speak two true-voicing languages, Basque and Spanish. We also describe the voicing system in Gipuzkoan Spanish and Gipuzkoan Basque, which is the closest dialect to Standard Basque and it has not yet been investigated experimentally. Twenty Basque-Spanish early bilingual speakers of Gipuzkoan dialects participated in two picture naming tasks. We described their voicing system by measuring voice onset time (VOT) in both Gipuzkoan Basque and Spanish, and used linear mixed-effects models to investigate between-language production differences. Our results show for the first time that adult early bilinguals who speak two true-voicing languages produce language-specific VOT in ‘voiced’ plosives. This finding demonstrates that bilinguals’ phonetic systems during production are more fine-grained than previously assumed, and contributes to a deeper understanding of granularity in early bilingual phonetic systems.