Modern information systems have to support a variety of different device types like desktop computers, smartphones, or tablets. Furthermore, it is important to enable users to use device types that fit their needs or are suited for the tasks at hand, e.g., allowing them to use multiple devices in parallel or sequentially by switching from one device to another. Such cross-device interactions must be taken into account already during requirements analysis to ensure that they are properly addressed in later phases of development. Unfortunately, current requirements modeling techniques do not provide adequate techniques to model cross-device systems. In this paper, we present an extended form of use case diagrams able to model such systems. Using our approach it is possible to specify which device types can be used when performing a certain use case and what kinds of cross-device interactions are supported. Based on this, we show how this information can be refined by integrating extended use case diagrams with our existing approach to model cross-device interactions in process diagrams. Thereby, we explain how requirements can be modeled visually in a model-based development process for cross-device systems.