I would like to have all email that come in to domain.com that are not listed as a user (catch-all) be sent to 2 locations. We want to use this format so both operators of the site can see/respond to emails about the site.

Bottom line: anything@domain.com forwards to e1@domain_x.com and e1@domain_y.com

I know we have to edit a file but I don't know which one.
Thanks for your help.

You need to write .forward file