If with 'app' you mean two different FuelPHP installations, then direct access is never possible. They are not aware of each other.
Within the same app, it should be possible (providing the packages/modules are loaded, namespaces are known, etc), but it is frowned upon, it creates tight coupling between two not-coupled application components, which should be avoided at all costs.
Instead, do a REST call (if another application) or an HMVC call (if the same application), instead of calling the model class directly, to retrieve the information.