Vexen gives the Twilight Town card to Sora after they 'fight', and tells him that the other side of his heart knows this place (obviously meaning Roxas). THIS is actually what Marluxia orders Axel to kill Vexen for, even after Axel comments that if Sora just blunders in one side of Twilight Town and out the other without realizing anything, they can still smooth things over.
WHAT'S MORE, the person who gave the Twilight Town card to Vexen in the first place... was Axel.
Where he gets it from is anybody's guess, but i suppose it could be made from his own memories as easily as from Roxas's. you could even make the arguement that it's from Sora's, since he must have Roxas's stuff in him someplace, but in all other instances there's a big song and dance about sampling people's memories for cards, and here nothing, so I'm leaning towards that not being the case.
It's impossible to tell what Axel wants out of this (well, the obvious answer to that is 'Roxas', but i mean in this moment) whether it's just to fuck with people or to cause dissention among the Castle Oblivion crew or something else entirely.
I will tell you this though, that Axel tells Namine explicitly that he doesn't believe the Organization's plan can work, so if he's trying to get himself and Roxas hearts, he's exploring other means to do it.
I haven't got to a point where the Roxas/Sora timeline is explained to me in any sort of detail, but it seems to me that in CoM Axel is using a lot of people, particularly a very willing Vexen, to see if there's some way to get Sora either to know what Roxas knows, or to have the Roxas personality take over entirely.
I guess there's no way to find out other than to keep going, though.
Careful with the KH2 spoilers, as usual (I'm a bit past Xaldin, if that helps).