I've come across two third cousins, who are third cousins through two different routes as the same two families had two separate marriages between them that leads to this situation.

I was thinking perhaps calling them `double third cousins`, but would like to know the official terminology.

You can see [an example of this relationship here](https://www.wikitree.com/index.php?title=Special:Relationship&action=calculate&person1_name=Ford-5719&person2_name=Lang-3140).  Notice that there are two sets of married ancestors, the Brown/Hodges marriage and the Brizendine/Cochran marriage, both common relations make them third cousins.  See popup menu that allows navigation among the four common ancestors in this screenshot:

[![Screenshot of popup menu for selecting which of the four common ancestors to view the connection through][1]][1]

  [1]: https://i.sstatic.net/YyRaA.png