In multiple animal taxa, including many birds and primates, members of mated pairs produce coordinated acoustic displays known as duets. By observing the behaviour of territorial animals as they respond to playback-simulated duets of rivals, we can gain insight into the behavioural significance of vocal duets. Playback experiments, however, have been conducted across a very narrow range of duetting animals. Furthermore, many studies have been conducted with single-speaker playback, whereas stereo-speaker playback offers more spatially realistic simulation of duets. Moreover, by evaluating the reactions of animals to separate loudspeakers broadcasting male and female duet contributions, we can study the interactions of both males and females with same-sex vs. opposite-sex rivals. We used a paired experimental design to broadcast duet stimuli through a single-speaker and a stereo-speaker apparatus to 30 pairs of duetting barred antshrikes Thamnophilus doliatus in Costa Rica. Our goals were (1) to evaluate whether territorial antbirds respond more aggressively to male vs. female duet components and (2) to assess aggressive responses of antbirds towards single-speaker vs. stereo-speaker playback. Neither males nor females differentiated between the loudspeaker simulating the male vs. female duet contribution during stereo-speaker playback trials. Barred antshrikes displayed significantly stronger responses to stereo-speaker playback compared with single-speaker playback. Males displayed stronger playback responses than females with closer, quicker and more vocal responses. These results provide evidence for a joint resource defence function of antbird duets given that pairs responded together with equivalent intensity to male and female simulated intruders. This is the first study to show that although duetting is an aggressive territorial signal, birds do not necessarily respond to sex-specific components of duets. Our results support the idea that spatially realistic stereo presentation of duet stimuli is critical for experimental duet research.