You may want to enable the automatic management of content when switching to/from double masters.

On the General subtab of the Extra tab on the Preferences dialog box, tick Auto-split/join content when dealing with double masters.

This experimental functionality attempts to automate the steps, explained in Convert two single pages to a 'fake' or 'real' spread and Convert a spread to two single pages.

Note

This functionality uses the variable F_AutoMoveFullSpreadContent.