Microsoft Word allows you to setup mirror margins. As the name would imply, mirror margins are used for double-sided documents such as those found in a book or magazine. With mirror margins, the inside margins are the same width and the outside margins are the same width.
If you are setting up facing pages for double sided documents, you can set up mirror margins using these steps. From the File menu, click the Page Setup option. On the Margins tab, use the drop down arrow beside Multiple pages, select the Mirror margins option and click OK.