Reflecting one of the state-of-the-art issues in SLA, this study aimed at inspecting the correspondence between Persian and English writing skills to see whether or not they emerged from a single entity called multi-competence (Cook, 1991, 2016). Besides, the impact of genre on Persian and English writing skills was examined to explore if the skills and their possible source were genre-sensitive or not. In so doing, 50 MA TEFL students, at the intermediate level of language proficiency, were selected through Oxford quick placement test. Each student wrote down four argumentative and descriptive compositions: two in Persian and two in English. Then the compositions were evaluated based on the criteria of IELTS task 2 writing scoring scale. To analyze the obtained data, Pearson product-moment correlations and one-way repeated measures ANOVAs were employed. The findings illustrated that there were significant correlations between Persian and English argumentative and descriptive compositions, which supported the possible existence of the single entity, multi-competence. The findings also displayed that genre affected the participants’ L1 and L2 writing skills, implying that the skills and their source, multi-competence, were possibly influenced by genre. Based on the findings, it is recommended that L1 writing instruction be included in L2 writing courses as they are inter-dependent and can help each other’s amelioration. It is also suggested that a genre-based approach be utilized in L2 writing instruction to enhance the students’ awareness over the building blocks of each genre.