Owners of the Xbox 360 version of Street Fighter X Tekken may have noticed that one feature is missing from the game, local co-op online play. This is when 2 players on the same 360 play as a team online. THe feature shows up on the PS3 version, but not on the 360 version. Street Fighter X Tekken assistant producer, Tomoaki Ayano , explained that due to time and resources issues. He continued to explain that the PS3's architecture made the feature easier to implement.
The rebuttal to this point, as many people have already stated, is why this is easily available on Mortal Kombat 9 for both systems without much a problem. Word is that the issue was due to how gamertags worked, but again, another company was able to add the feature. Capcom's Seth Killian tweeted that will have another update regarding some of the Street Fighter X Tekken issues soon.
Source: Capcom Unity