According to reports and rumors:

One game is Street Fighter x Tekken (SF "cross" Tekken) that plays as 3D models in a 2D plane. Play style similar to SF4, with the Tekken characters either gaining projectiles or modifications to their moves to destroy/break through projectiles.

The other game is Tekken x Street Fighter (Tekken "cross" SF). This one would convert the Street Fighter characters into full 3D for a more Tekken-style game. There are nearly no other details on this game.

Which company works on which game is in Marik's post.