Updated: 2 player Street Fighter X Tekken co-op mode on Xbox 360 won't be fixed, problem on Microsoft's end
You can do it in Halo, Gears of War, and countless other titles. Stay classy, Capcom. Deflect blame where addressing an issue would just be too damn hard.

Capcom explains why Street Fighter X Tekken's 12 DLC characters were on disc, fighters available in the fall
"For example, not buying costumes in the Street Fighter IV series means you will not see the costumes when playing another person who did buy them", except those costumes were also found on disc like with RE5's versus mode which was released next month.

Street Fighter X Tekken's Mega Man and Pac-Man available for download on March 13th
(for free)