Sources with several publishers
In the reference list
If a source, e.g. a book, has several publishers, they are divided with a semicolon in the reference list.
Example:
Schmid, H.-J. (2017). Entrenchment and the psychology of language learning: How we reorganize and adapt linguistic knowledge. De Gruyter Mouton; American Psychological Association. https://doi.org/10.1037/15969-000