Having two pointers/mice on the same computer
You can connect multiple mice to share the same pointer.

Multiple pointers is a bit different, research and development is still ongoing in the Linux world that you may wish to peruse for information:

Video example of using multiple pointers:
http://www.youtube.com/watch?v=b2iYNfl-2Es

http://en.wikipedia.org/wiki/Multi-Pointer_X

Here is, more of a demonstration site for OSX:

http://www.dualosx.com/dualosx_en.htm