Co-authoring and Box for Office each create "places" in Office applications, which is confusing to the end user. Per Box Support, files opened via the Box For Office integration cannot be co-authored.

The Box for Office integration includes Office ribbon icons, more intuitive file open/save workflows, and has been part of the Box integration environment for years. Consider either improving the UI of the "Place" created by the co-authoring integration or allowing co-authoring on content opened via the Box for Office integration.