Multiple mouse support is ONLY available in the Linux port and is NOT planned for Windows or Mac. Each player may control balls although only one can scroll the screen.
As an unsupported feature, the Linux version of World of Goo can be played with up to four players at a time, each using a separate mouse. In order to use this feature, edit config.txt and change the "manymouse" parameter value
from "false" to "true".
Furthermore, the mouse event devices for the mice to be used must be readable by the user running World of Goo. An easy way to set this up (assuming udev is used for the /dev filesystem) is to execute (as the root user):
chmod a+r /dev/input/by-id/*event-mouse
Note that this change is not permanent (permissions are reset when the system is rebooted) and may compromise user privacy on multi-user systems.