Satellite missions demand ever greater connectivity, especially in the LEO regime. In this paper, we introduce the new mega-constellation services in space paradigm: we show that megaconstellations, deployed to offer innovative services to Earth's users, can provide excellent connectivity to LEO spacecraft as well. First, we characterise the communication link between space users and the actual OneWeb and Starlink constellations. A full set of results in terms of availability, access duration, Doppler, and path losses as a function of user orbital parameters, identifying optimal user orbits, is provided. The results achieved by a multi-system user able to communicate with both fleets are also presented. The potential improvements available if geostationary constellations are used to complement LEO megaconstellations in a multi-orbit system are discussed as well. Finally, we focus on two LEO use cases: the International Space Station and an Earth Observation Sun Synchronous satellite. All the results demonstrate the numerous advantages of the mega-constellation connectivity solution, which can transform LEO spacecraft into highly responsive nodes of a space-to-space network.