To set this go to your control panel and find in sidebar XML Sync link. Once master is set you can copy master url into other control panel.
To summarize: All you have to do is to mark one website as master and in other website insert master URL like: http://www.yourmasterwebsite.com
From there you will have option to choose which properties to import and which to skip. Once property is skipped it will be missed in the next sync. Please be patient this can take some time depending on number of properties and images per each property.