Updated: 2 player Street Fighter X Tekken co-op mode on Xbox 360 won't be fixed, problem on Microsoft's end
http://www.capcom-unity.com/ask_capcom/go/thread/view/7371/28972471/Will_2_player_local_co-op_%28on_XBL%29_be_patched_in_for_the_360_version_of_SFxT&post_num=9#517227135 "It will not be patched and apologies for the manual inclusion." According to the team: "PS3 version: It is possible to mix and match online and offline players for scramble battles and pair play battles. Two local players can play on the same PS3 using 1 PSN ID, and play against other players online. I think you need to find a better team. If one fighting game can do it, pretty sure another can.