Items can have more than one parent but you need to add some custom modifications. In the content types that need to refer to your specific content type add guids or array of guid fields that keep the item ids of the content type they refer. You can use this blog post
to help you get started. It requires some custom development but still you can get some pretty nice results. We will definitely have this implemented for future releases but for now using sitefinity backend UI you can only set up items that have one parent.
To speed up the development process I suggest you to use Sitefinity Thunder
Visual Studio extension.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items