When you change a two-page master page to a single-page master page, the left page of the two-page spread is deleted. Any publication pages to which the left page of the two-page master page was applied will display the elements from the right page.
On the View menu, click Master Page.
In the Edit Master Pages task pane, click the arrow next to the two-page master page that you want to change to a single-page master page, and then click Change to Single-page.
When asked if you want to continue, click OK.