This letter studies an unmanned aerial vehicle (UAV)-enabled two-user interference channel, in which two ground users send individual messages to their respective UAVs in the sky. By considering probabilistic line-of-sight (LoS) channel models, we jointly optimize the two UAVs’ three-dimensional (3D) positioning locations and the two users’ power control, to maximize their sum-rate throughput. Although this problem is non-convex and difficult to solve in general, we obtain the optimal solution by decoupling the positioning optimization and power control. The optimal solution shows that if the signal-to-noise-ratio (SNR) $\rho$ is high, then the two users should take turns to transmit at the maximum power in an on-off manner and each UAV is positioned exactly above the associated user. By contrast, if $\rho$ is low and/or the two users’ distance $D$ is large, then the two users should transmit simultaneously at the maximum power, and each UAV should be positioned away from the two users for interference mitigation.