How can I send RPC from one client to another client directly in multiplayer game?

Actually, I am developing a multiplayer game in which I create a server for 8 players in tournament style i.e. four one vs one game on level one and two one vs one game on level 2 and so on. So i have to create a match between two players out of all 8 players connected. So I need to know how can I establish communication between those two clients. I really appreciate your help.