@Charles, to put it simply. Does it give you an unfair advantage that is not known to any other player in the game you are joining or hosting? If yes then it would be considered cheating, as it negatively affects the experience of all other players in the game. I'm sure you have seen the endless legions of unhappy people posting on the forum about this.
Additionally, functionally there is no difference between a player using 2 accounts and 2 seperate people working together every single game in order to win.
There is always the "Play with Friends" game mode that is designed for friends to play. But, I'm sure you will say "But I can only play against AI in that?" To which my answer would be, idea that you want to gang up on the real unsuspecting players is just mean.
@Charles, if its a random person in the game then its not cheating. If its someone you planned to go into the game with, it is. It is very easy to tell when you have played dozens, if not hundreds of games with the same person.
Briand, I think part of that could be resolved in the game if you could chat with Allies but I can understand why that might be a bad idea. A better strategy for taking someone's last man in a territory or area is allowing them to get out of there first; they move away to keep a strong hold, and you move in. That's fair but taking a territory that might have otherwise been a strategic position or got them to the other side of the board, that's actually kinda a dick move but also understandable in the current way the game works. I think it also depends on many other factors, like how many armies they have and where they are positioned. Many of the problems like this are due to lack of communication. And generally while I don't like alliances that much, I think we've all been on both sides of what you mention. Best!
After reading through this thread, I have only one thing to say.
Charles...I feel really bad and ashamed for you. I hope Risk bans both you and your "friends" accounts.