Two separate fighting games; one built on the Street Fighter engine and another on the Tekken engine. The games are called Tekken X Street Fighter and Street Fighter X Tekken, utilising only characters from those two franchises.
Street Fighter X Tekken, is a fighting game that will feature both Tekken and Street Fighter characters on a 2D plane with EX-esque super moves and the art sytle of Street Fighter 4. The title is being developed by Capcom.
Tekken X Street Fighter, is the same concept with different gameplay. Street Fighter characters are fighting in Tekken's world, which will feature 3D framework fans know from the series, and this title is being developed by Namco.