If you want to concatenate two elements in a Numbers spreadsheet and include a line break between them, use CHAR(8232)` to represent the line break. For example, putting a line break between the contents of cell A1 and B1 would look like

=A1 & CHAR(8232) & B1

If you were in Microsoft Excel on a Mac, the analogous command would be CHAR(13); on a PC it would be CHAR(10).