Marvel vs. Capcom: Clash of Super Heroes

The storyline is loosely based on the X-Men: Onslaught arc from the Marvel Comics continuity: following an incident in which mutant telepath Charles Xavier was forced to enter into the mind of Magneto, part of Magneto's anger slipped into Xavier's psyche, and after merging with Xavier's darkest emotions it spawned a powerful psychic entity known as Onslaught. The game expands on the story by having Xavier's conscience calling forth heroes from the Capcom universe in order to help stop him.

Résumé

This PlayStation port differs from the arcade version, in that players are unable to switch characters on-the-fly, reducing the secondary character to an assist role only.
Significantly, this removes the tag team aspect of the original.

It instead offers a "Cross over" mode where each player chooses one character, and the second character for each team is a mirror of the opponent's pick. In this mode, the players may switch characters at will, but the teams must always be identical.

This compromise is most likely necessitated by the console's small memory size, as the use of identical teams alleviates the memory requirements.