These assorted Youtube videos show some of the diverse co-op mode gameplay available for iPhone, iPod Touch, and iPad.
Spider: The Secret of Bryce Manor HD for iPad
$.99 (video by tigerstylegames)
Minigore
$.99, HD version , $1.99 (video by TimoVihola)
Tilt to Live
Free with $1.99 “Viva la Turret” Co-op Expansion, HD version Free with $.99 “Viva la Turret” Co-op Expansion (video by xlogicalzx)
Forbidden Island for iPad
Minecraft – Pocket Edition
$6.99 (video by JackFrostMiner)