"I think what you're asking is "is metagaming illegal" and the answer is yes"
If the answer is yes, why is it allowed to continue? According to president pericles, it's also been going on for some time.
If two people cooperate every step of the way and there is no
indication they will ever turn on each other, you may as well say that it's a multi-account due to it being played exactly like one. On the other hand, having looked at their profiles, it doesn't seem as though they are cleaning up by employing that strategy.
Here's another variation of the same problem, which leads to a question. A person enters a game and discovers through messages that they have the same likes and dislikes, share the same philosophy concerning the game etc, with their neighboring country. They ally, and having the advantage of trust, win the game. They conclude that it worked so well they ought to join another game and do it again.
But since they do not know each other in the real world, is it considered meta-gaming?
Denzel73 said: If you make friends with someone DURING a game, and your alliance wins, than Diplomacy had some meaning.
So the two players could argue that they are merely bringing some meaning to every game they play :)
As for me, If I see those two had signed up for a game, I wouldn't play in it.