You play the player, not the board, so to speak. In this scenario, if you know you have that allying with him in both games will help you with the win, then do it. This is a game of winning at any cost.
People take meta-gaming to a degree that it shouldn't go. What I mean by that is that there are those out there that feel that using people's posted information on a forum or info about how they have played previous games is considered poor form meta-gaming. Personally, I see issues like these (and the one you're talking about) as finding a way to play the player in a way that best suits you.
Here's another example: if I'm playing an anonymous game and I know the players in the game but not who's who, and someone in the game sends me a message while I'm on (indicating they're on also), I have no problem going through the list of people in the game and checking to see who's on to be able to identify which player it is I'm talking to. I can then go through their profile and look at their play style. Sure, it's bringing outside information into a game, and yes it can influence the way I play, but it's by no means doing something beyond the framework of the way the game is designed.
When I look at whether or not something is "legal", I compare it to things people do in FtF games. People have listened into other people's conversations before, placed extra units on the board that don't belong (flying dutchman), ask other players about someone they've never played before, people pay attention to how well a player is doing in a tournament to determine whether or not they should ally with the guy. I've even heard stories of how one player took a set of orders from the box so that when orders were read, that opponent didn't have any official orders and all of his units were reduced to "holds".
The only thing I won't really ever do is (a) ally with someone simply by virtue of them being a friend and (b) use multiple accounts to play multiple countries. Anything outside of that is fair game.