Sandboxes are locked by a particular SCM client instance (Eclipse Client, VS Client, CLI) preventing other client instances from working with them. This doesn't prevent other non-SCM programs from accessing those files and making changes (e.g. text editors).

When using external programs there are extra precautions that should be taken to avoid data loss (e.g. enabling the source control shed, refreshing before performing SCM options such as accept and check-in). There is an article on this topic:

https://jazz.net/library/article/531/

That being said, sandboxes are generally assumed to be used by only one user there can be only one person on one RTC client that accepts new changes, merges conflicts, checks-in new changes. Perhaps if the team picked a single person to manage these SCM tasks and they were in tight collaboration it might work. This person would need to be fully aware of what files the other person has changed so that they can check them in, associate work items and perform merges on their behalf.

This is starting to sound alot like pair programming. If this is in fact the goal then both users could work in the same client/IDE with no need for shared sandboxes.

Otherwise, it might work alot better if each team member has their own sandbox and repository workspace.