I was told by a CBP agent that customs declarations can only be for more than one person if there is a legal relationship such as marriage, children, etc.
I was also told by the same CBP agent that a non-resident spouse of a U.S. citizen could use the line for US citizens/US residents if she or he was traveling with the spouse.

I would suggest that an S.O. does not qualify as a spouse and that entering together to ensure that if there is a problem the U.S. citizen can assist the non-U.S. citizen is probably not a good idea. The immigration hall is not the best place for ad hoc advocacy and engaging in such activity could quickly backfire.