Panel a: velocity-integrated [CII] maps of #C and panel b: #D, obtained by collapsing the spectral channels corresponding to a 2 × FWHM interval centred on their z_[CII] (see spectra in Fig. 4a, and values in Table 1). The velocity ranges used to collapse the datacubes are shown in the top-left corners. The rest-frame UV centroid of #B is indicated with a plus symbol. The dashed blue ellipse defined in Fig. 3 (see Sect. 3.3) is shown for reference. The grey solid line and the black contours indicate the significance level of the [CII] flux emitted by the full system as defined in Fig. 4b. The green solid (dashed) contours indicate the positive (negative) significance levels at [3, 4]σ of the [CII] flux maps of #C panel a and #D panel b. To help the visualisation of panel b, we used a maximum value of surface brightness in the colour-map range as defined by excluding the northern bright spot (corresponding to a fraction of the emission of #A). The ALMA beam size is given in the bottom-left corner. North is up and east is to the left.