The present study analyzes the influence of L1 on the learning of the lexicon of two L2s. While a number of studies have been conducted on the influence of first language on the learning of one second language, little work has been done on the influence of first language on the acquisition of two second languages. The data for the present study was collected from the Pakistani learner of English and Arabic as second languages in a madrassa named Dar-ul-Uloom Mhammadia Ghausia in Gujrat. Cummins’ (1980) model of Common Underlying Proficiency (CUP) and Separate Underlying Proficiency (SUP) was used as the theoretical framework. The data showed that Urdu played a largely facilitative role in the learning of the lexicon of both English and Arabic; however, there was a difference of the degree of facilitation. Due to the inherent similarities between Urdu and Arabic, Urdu played a more facilitative role in the learning of Arabic lexicon; whereas, due to the structural differences between Urdu and English, the former played a less facilitative role. Therefore, the learners found learning English more difficult than learning Arabic and the types of errors they made also proved the same.