This study examined how emergent bilingual children's English and Spanish proficiencies moderated the relationships between Spanish and English input at home (bilingual home language input [BHLI]) and children's oral language skills in each language. The sample comprised over 1,400 Spanish-dominant kindergartners in California and Texas. BHLI was estimated by parental report of what language adults (mother, father, and other adults) and peers (older and younger siblings and friends) usually speak to the child at home and the language in which the child watches TV programs. Spanish and English oral proficiencies were measured by the Woodcock Language Proficiency Battery-Revised. Although a "time-on-task" model suggests straightforward linear associations between language use at home and oral skills within the same language, we found far more complex cross-linguistic relationships even when controlling for school-level random effects and individual-level confounds: (a) children's proficiency in a language (English or Spanish) moderates the relationship between BHLI and proficiency in the other language and (b) BHLI moderates the relationship between children's Spanish and English oral proficiencies. The results suggest that among this population of children, bilingual home language environments that maintain high levels of Spanish use are associated with additive bilingualism whereas bilingual home language environments with high levels of English use are associated with subtractive bilingualism. However, the cross-sectional and correlational design of the present study requires caution in drawing simple conclusions about the relationship between BHLI and children's oral language skills.