The Pro version of PublishPress Series allows you to add posts to more than one series. The core plugin only allows posts to be added to one series.
This feature can be enabled by going to Settings > Series Options > Pro Features in your WordPress admin area.
The most obvious change you will see after enabling is inside the post editing screen. The “Series” metabox will allow you to choose more than one series.
You will also see a similar change on the “Quick Edit” screen for posts:
Please note: if you want to try Multiples, please start on a test site as it's not easy to go back to using just PublishPress Series. Your posts will lose all their series information because this feature significantly changes the data structure of series when activated. You will have to re-associate your posts with series again. The series will still be there, just the post-to-series connections will be lost.