Watermark in each column of a landscape two column document?

While what Hans says is correct, the proper solution for this particular situation is to use the "2 pages per sheet" option in the Page Setup (not Print) dialog. You can learn more about this at http://wordfaqs.ssbarnhill.com/2PagesPerSheet.htm

1. Start by converting the document to a single column (you can retain the landscape orientation).
2. Double-click the top of the horizontal ruler (or use the dialog launcher in the Page Setup group on the Page Layout tab) to open the Page Setup dialog.
3. Under Pages: Multiple pages, select 2 pages per sheet.
4. You'll see in the preview that you get two pages side-by-side on one sheet of paper. These virtual pages will behave exactly like a real page; you can use watermarks, page borders, columns, etc.
5. If your program has more than four pages (front and back of a single sheet, folded), then you should use the "Book fold" option instead. The document will look exactly the same but will print in the correct order for creating a folded booklet.

Another way of achieving your aim would be to leave the document in Portrait Orientation (perhaps increasing the font sizes) and then use the Two Pages per Sheet option when you print the document.

You may be able to create a "watermark" effect on each column however many columns you have.

1. Use the Text Box feature.
2. Insert a Text Box on one column. 
3. Use diagonal, vertical or however text direction you wish to create.
4. type in your text and use "rotation" option.
5. Right click the Text Box and click "options" / Advance Layout and click overlap. (Position Tab)
6. Now, Click Text Wrapping Tab and click "behind text" on Wrapping Style
7. Close Advance Layout
8. High Light the Tex Box and Text
9. Click Home Tab on Tools and pick/ click text "light" colour you prefer.
10. if you're satisfied with the looks, copy and paste for each column. 

Hope this will help you. Good luck!

While the "2 pages per sheet" setting is a better approach, you can also do this by anchoring the watermarks to the page header in the appropriate location. It's very difficult to create a watermark that replicates the one that Word creates, so the best way to start is to insert a watermark from the Page Layout tab, copy it, remove it, and then paste it twice in the desired location (resizing as required).

Watermarks "live" in the page header (or footer), they don't know anything about columns. So you'd have to create two watermarks side by side, one behind each column. Word does not provide for this automatically, you have to create/edit the watermarks yourself in the page header.