If you were wondering why only the Xbox 360 version of Street Fighter X Tekken is missing the four-player cooperative mode (2 local players online vs 2 local players), Capcom's Tomoyaki Ayano has an answer for you.
Unfortunately, due to time and resource issues, we weren’t able to get that feature into the Xbox 360 version and we apologize to those players who were looking forward to it. Throughout the development process, there are times when new features are added (and we added a lot of them!) and there are also times where we have to make tough decisions regarding things to cut. This was one of those times.
The PS3 version has this mode; Ayano says that they found the architecture of the PS3 to be "more conducive to implementing it." I'd bet that pisses some people off.
Ayano teases that they'll have more information on new updates for SFxT soon, so stay tuned. Maybe a fix?