The two columns represent the left and right channels of a stereo recording. The reverberator object is intended to mimic an acoustic environment where the time-delay between your ears carries important information.
Depending on what you want to analyze by plotting, you can either convert the audio signal to mono by taking the mean, or plot each channel separately.
To add multiple reverberations together, you can take the means of the respective channels. To be clear though, the reverberator object does not just produce a single reflection. It includes a reverberation tail, as described in the algorithm section.
