Leo says that programmers use apps that have "version management". People can "check in and out" the files to edit them so they don't end up colliding with each other's updates. Leo recommends GIT. Another option is Adobe has Version Cue for Designers.
Leo also recommends using Network Attached Storage. This would be attached to the network but not to either computer and would act as a central place to store files. He could use versioning to prevent Tim or his brother from overwriting each other's work. A Pogo Plug or Drobo would work.
Check out Tim's Comic Books at Strips4.com - Look for MAPS, about a kid traveling between dimensions.