Our study examines chunking ability in the processing of auditory multi-word expressions (MWEs) in first (L1) and second languages (L2) using a single- and dual-task paradigm. The findings reveal that divided attention caused by dual tasks does not impair L1 speakers’ ability to bind individual words into a complete MWE (i.e., a unitary chunk). In contrast, L2 learners struggle to form complete MWEs under dual tasks, representing them as smaller, multiple chunks in memory. Divided attention also reduces the overall number of chunks recalled for both groups. Additionally, increased language proficiency and repeated practice through training are positively correlated with the formation of larger chunks, whereas greater working memory capacity is associated with the recall of a greater number of chunks. These findings underscore the challenges L2 learners face in acquiring relatively large chunks and suggest that L2 learning improves through gradually binding smaller units into larger chunks over time.