You can insert a horizontal rule using the Menu: Select Menu Insert
-> Horizontal Rule...
.
Alternatively, you could use paragraph styles:
- Select the paragraph between the two frames and open the paragraph style options (using Menu
Format
->Paragraph...
or the context menu: right-click and selectParagraph...
). - In the paragraphy style options, select the
Border
tab (1) and add a bottom border by clicking in theUser-defined Line arrangement
area as illustrated (2):
Finally, you could just modify one of the two frames and add a border at the frame's bottom or top respectively.