In the context of Earth observation constellations, we consider the problem of allocating orbit slots to clients requesting some ownership of orbit portions overflying desired regions on Earth. This problem arises prior to operational scheduling of observation tasks, in constellations where users can directly communicate with the satellites using their own ground stations. Observation scheduling in the exclusive slots is then delegated to the clients themselves. To perform the allocation of exclusive slots, we propose a two-level optimization approach, where the optimization process (led by either utilitarian or fair criterion) explores the solution space using a feasibility checker based on a constraint solver. We experimentally evaluate and analyze their performance on randomly generated order books and real constellation configurations.