Based on multicompetence theory and the framework of adaptive transfer and to identify the development of multicompetence, this study investigates how and when 15 Chinese EFL writers with different levels of L2 proficiency and exposure to (L2) instruction used their language repertoire when they composed argumentative essays aloud in English L2 and Chinese L1. To avoid the effect of translation, the participants composed English essays first and Chinese essays a week later. Through the transcribed think-aloud protocols, the specific purposes for which the languages were used were categorized. The results indicate that regardless of L2 proficiency and exposure to (L2) instruction, extensive L1 use was evident in both L1 and L2 processes; in the L2 process, the participants were capable of using both Chinese and English to complete the composing activities that helped them generate the content of L2 papers and cope with language issues. In the L1 process, higher proficiency and experienced writers’ use of language repertoire was manifested in a more bilingual way than low proficiency and novice writers’ usage. Examining the L2 process presented less uniform and more complicated methods used by the participants to engage in language use. While higher proficiency and experienced writers verbalized in both L1 and L2 to perform more cognitive-related activities, low proficiency, and novice writers used a mixture of L1 and L2 in the activities that were more related to the compensation for their L2 linguistic inadequacy. Subsequently, a picture of the development of multicompetence was demonstrated. The merged system of L1 and L2 from separate to overlapping was observed during the composing process across languages; those overlapping parts were usually discovered in most of the areas more related to linguistic or lexical problems. The more the L2 writers’ superlanguage system is merged, the more developed their multicompetence, and the more flexible they become in using language repertoire to write. L2 learners’ flexibility with using language repertoire should be the indicator of their approaches to language use for composing activities. Corresponding to the framework of adaptive transfer, such L2 learners’ flexibility grows with the development of multicompetence, which is affected by L2 proficiency and especially exposure to (L2) instruction. This study challenges previous studies on the place of L2 proficiency in L2 writers’ language use and provides a new perspective on interpreting L2 writers’ language use during the composing processes in L1 and L2.