Due to the demanding development of new line corridors and complex approval procedures, new overhead lines increasingly carry several circuits bundled on a single tower, which raises the probability of inter-system faults. The protection-related treatment of inter-system faults has so far been described only insufficiently in the technical literature. In this work, inter-system faults are simulated on a dynamic network model that represents a faithful parametric reproduction of a 150 km double-circuit line. Subsequently, the behavior of conventional distance protection under such faults is examined based on the results. The investigation reveals a non-selective fault treatment, reflected in an underreach of the distance protection and an incorrect zone decision caused by large deviations in the calculation of the fault loop impedance. To address this problem, a newly developed calculation method for more precise fault location is presented, which takes into account the current and voltage values of both circuits involved in the fault as well as the zero- and positive-sequence coupling. The method is not restricted to identical circuits of the double-circuit line. Its accuracy is examined using simulated faults and compared with existing distance protection methods. The new calculation method achieves a clear improvement in distance calculation and zone decision.