There's no provision in this site to support private and inter-CC tournaments. Their announcements only point to the worldwide Clashes which they organize from time to time.
Still, it has been done frequently by private organisation extern of the company. Some examples are published in the forum to enable easy inter-CC communication.
There are websites providing general ladder systems for public use.
In addition, public webservices may help by holding common documents and tables between the CCs. Verbal communication, e.g. by Skype or TeamSpeak, adds to the fun.
You'd have to set up your mind in your CC and agree between the CCs about number and skills of the participants, forms of play and scoring of the results. Last but not least, which wager, or none?