Use columns
This assumes that the dividing line is provided elsewhere.
Begin with a blank document, switch to 'print layout' view and select Format > Columns…
Try the following settings (assumes six inches between margins — as you change one width, the others will change to fill the page):
- Number of columns: 2
- Equal column width: unchecked
- Width 1: 7.5 pi
- Spacing 1: 1 pi
- Width: 2: 27.5 pi
- Apply to: Whole document
Then enter your left-column text and format to flush right. Insert a column break after the text (Insert > Break > Column Break). The cursor will now be at the top of the right column. Enter your right-column text and format to Flush Left.
![At top, Columns dialogue box; at bottom, resulting word document.](https://cdn.statically.io/img/i.sstatic.net/wbgAd.png)
NB, checking the 'Line between' box provides a visual placeholder. I used picas here but use whatever suits you best.
If this formatting should not apply to the 'whole document' then at the appropriate place, insert a 'section break' and then set that section to 1 column. Note that after adding another section, this section's column formatting will change to indicate that it applies to 'this section' instead of 'whole document'.