In StoriesOnBoard one subscription belongs to one workspace. If you'd like to merge your workspaces in one subscription then please move your story maps into one workspace. There's a short guide on how to do it here: How to move maps between workspaces.
The maps can be separated with permissions within the workspace, they don't need to be accessible (or even be seen) by everyone. For example by using collections.
After moving into one workspace you can subscribe as described here: How do I subscribe to the paid service?