If there's more than one publisher and as far as you can tell, they're all equal partners in producing a work, name all of them separated by forward slashes ( / ).
For example,
United States, Supreme Court. Obergefell v. Hodges. 26 Jun. 2015. Oyez, Legal Information Institute / Justia / Chicago-Kent College of Law at Illinois Tech, www.oyez.org/cases/2014/14-556.
Book publishers often have imprints -- smaller brands within the overall publisher. When citing, you'll typically only include the publisher name most directly responsible for the book; in other words, the imprint name and not the overall publisher, even if both are mentioned somewhere.
For instance, Orb is an imprint of Tor. When citing a book published by Orb, you won't mention Tor in the citation.
Silverbeg, Robert. Dying Inside. Orb, 2009.
