Multiboxing, what to do about it? happening to me right now

Hey guys. Im in my first game of Subterfuge and i really love this game. Im doing really good and had an alliance with a guy and another player seemed pretty confident but was in a bad position. Another player NEVER responded to me and he never talked to the guy i was allying with either. I already had a bad feeling because my enemy told me that i will regret my actions and he always asked me to team up agains my ally but never to attack the non responsive guy. As i said i already got a bad feeling about this and out of nowhere the non talker attacks me right after my enemy told me i should resign. So i called him out on "nice two accounts you have" and he actually responded with "what do you mean two accounts?" FROM THE OTHER PLAYER. So i dont know if he did this on purpose or accident but nonetheless he just confirmed that he is playing with two accounts = cheating. I can of course provide screenshots if you guys dont believe me but i will not do it in my first post because i dont know if its against the rules. Im just really pissed right now because i really like this game and actually did good in my first game and now my enemy has another account in this game. Do you guys have encountered similiar problems? Im lost right now. Edit: And now he ist talking to me that i shall resign from both accounts.. Edit 2: and he has 20 games on his "main" account (the one hes always online with) and 18 with the other so im pretty sure hes doing it all the time... I just dont get why people are doing this.. its only a 4 player game im in and he has 2 players. How is it fun.. i was concerned about people doing this before i even started and now its happening in my very. first. game.