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 workspaces and opens the workspace.
You can alternatively make a copy of the workspace, and continue editing there.
(1) Workspace in-use warning