Signal interference between two light detection and ranging (lidar) scanners can occur when the transmitted laser energy from one lidar is coupled into a second lidar either by scattering or direct transmission. By ray tracing lidar transmission paths, it is shown that signal interference can be modeled with the coincidence of intersections between two lidars’ optical axes and a scattering target or medium. A geometric analysis of intersecting transmission paths is presented for circularly scanning lidars from which theoretical interference risks between two lidars may be deduced. Using the geometric rules proposed here for interference, a Monte Carlo simulation is presented that models interference risks and occurrences. This geometric approximation simplifies analysis by ignoring radiometry for scanners within proximity to one another. Simulation results are used to introduce the concept of intersection point density from which one may assess areas of greatest interference risk for two-lidar, in-plane scanning lidar configurations. It is shown from the intersection point density that the risk for signal interference between two lidars exponentially increases as targets are located closer to and between the two scanners. Simulation results are compared to previously published experimental observations of interference to demonstrate the theory’s potential for interference modeling.