This feature requires the Pro version of the PublishPress Series plugin.
The Pro version of PublishPress Series has a feature called “Multiple Series” that allows you to add posts to more than one series. The core plugin only allows posts to be added to one series.
When this feature is enabled, the “Series” metabox will allow you to choose more than one series, as in the screenshot below.
You will also see a similar change on the “Quick Edit” screen for posts, as in this screenshot below:
This feature can be enabled by default in the Pro version of the plugin. It can be disabled by going to Series > Settings, then the “Pro Features” tab.
