Smartphones and tablet computer are ubiquitous in daily life. Many people carrying smartphones and tablet computers with them simultaneously. The multiplicity of different sized devices indicates the conflict between the maximal interaction space and a minimal bulkiness of the devices. This dissertation we extend the interaction space of mobile devices by adding mutual-spatial awareness to ordinary devices. By combining multiple mobile devices and using relative device placement as an additional input source we designed a mobile tabletop system for ad-hoc collaboration. With this setting we aimed to emulate the concept of so-called interactive tablecloth, which envisages every surface of a table top will become an interactive surface. To evaluate the concept we designed and implemented a working prototype, called MochaTop. To provide the mutual-spatial awareness we placed the mobile devices on an interactive table. For the future we believe in possibilities to replace the interactive table by technology integrated in the mobile device. In this study we used both one Android smartphone and one Android tablet as mobile devices. To track the position of the devices we used one Microsoft Surface2 (SUR40). The system is designed for exploring multimedia information and visual data representations by manipulating the position of two mobile devices on a horizontal surface. We present possible use-cases and environments. In a second step we discuss multiple low fidelity prototypes. The results are integrated in the development of MochaTop. The application MochaTop is designed as an example for exploring digital information. To influence the participants not too much by the content, we choose a common topic to present in MochaTop: coffee production and trade. We present the implementation of MochaTop and the conducted user study with 23 participants. Overall we could awaken interest for future systems by the study-participants and show that the system supports knowledge transfer. Furthermore we were able to identify design challenges for future development of mobile tabletops. These challenges concern mostly input feedback, interaction zones and three dimensional input.