Duetting is a cooperative behaviour that can have multiple functions. All 22 species within the genus Laniarius (Malaconotidae family) produce duets and solos with varying levels of complexity. We experimentally investigated the Yellow-breasted Boubou (Laniarius atroflavus), an endemic species from W African montane forests, which uses sex-specific signals for both solos and duets. Using two different experiments, we tested the response of territorial pairs, during the breeding season, to the playback of male and female solos and duets in order to investigate their functions in context of territorial intrusion. For the experimental intrusions of duetting pairs, we simulated the male and female songs at a close distance to each other as well as at a larger distance. The first goal was to assess whether intruder type affects the responsiveness of territory owners. The second goal was to examine how spatial segregation of intruding pairs singing duets affects the response. The majority of tested pairs responded to playback by approaching speaker(s) and singing. We found the strongest approach response was to the playback of duets and male solos, with the response to female solo being weak or having no response. The male-initiated, and led, duet was the most frequent and intensive vocal response, while female-initiated duets and solos of both sexes were less often observed. Regardless of the treatment type, the joint response of a pair was stronger if females were involved in it. Results indicate that in Yellow-breasted Boubous, males have the leading role in responding to intruders regardless of the sex or if they intrude the territory as a solo or as a duetting pair. Males were observed increasing their response strength when motivated by their mates, suggesting that female vocalisations are addressed to their own partner. Our results suggest that the most probable function of duets in the studied species is joint resource defence; however, mate guarding and the guarding of paternity are still possible. Our results are consistent with earlier findings showing that structural differences in male and female song of several Laniarius species can be the result of evolution under similar pressure from the need to defend resources together and the potential betrayal of a partner.