It's something that seems simple but I just couldn't find that option. I would like to see just one waveform (when the "LL" option is open on the timeline) in the audio layers to gain more screen space. I noticed that there is an answer to this question in the Premiere community, but no one talks about the solution for this in After Effects.
I dont believe there's any way to customize the way this is displayed. If you have a stereo audio file, it'll display both tracks. You could replace this file with a mono version, and it'll display only one waveform.