Each workspace can only be opened by a single user to avoid confusion and inconsistent data. If you try to enter a workspace that is already opened by another user, you see the dialog shown below. If a user is editing a model, all workspaces are marked as "in use".

Pressing "Yes, open" will kick the other user out of the workspace and opens the workspace.

You can alternatively make a copy of the workspace, and continue editing there.

If you frequently run into these problems, consider changing the structure of your workspaces and use shared scenarios (Sharing Scenarios) and submodels (Working with Submodels) instead.

(1) Workspace in-use warning