They did in Shadow the Hedgehog as a little secret. If there is a 2nd controller and Shadow has an assistant CPU like Sonic, P2 can use the 2nd controller to control the assistant.
Still, I am not very excited about co-op. I mean, you got through SM64 and Sunshine without an extra cursor thing, right? The game is based 99% on Single Player, and the experience probably does not change much with an additional cursor.