You can do this in two ways.
One is to use a Continuous break. For example, you are halfway down the 2-column page and need to insert your border.
Turn on "Display hidden characters" (Ctrl+Shift+8). At the end of your second column text at that point, insert Break/Continuous (Alt+I,B,T). Then type a return to provide an empty paragraph, then insert another continuous break. Select the empty paragraph and format it for one column. If you then make it right aligned, you can underline it or insert a border above or below it.

The seonc way is to use Insert/Shape and choose a line, which you can then format as you wish. Holding down Shift as your draw the line will keep it horizontal.