The USB audio adapter is input only and cannot be used to send audio from one player to the other.
WiFi interference is likely in a shopping center. Note that humans are basically bags of water and water absorbs WiFi. Therefore there could be a decrease in WiFi efficiency as the store fills with water bags. Elevating the router such that it is above the crowd will likely help. Don’t enclose the router in a dense cabinet or closet.
In your current setup each player connects wirelessly through the router’s WiFi, there is no direct WiFi connection between the units. The 10m distance between units is not significant as far as WiFi is concerned.