This is known and has been thoroughly researched. Please see the links below on auditory masking. The auditory system and brain can only process the dominant signal (or, can only process dominant signals very well). Back in the 1980s and 1990s this research was used to guide audio compression algorithms such as MP3 and Sony’s ATRAC in the MiniDisc system.
This is one reason why some people don’t care about audiophile grade sources or equipment.