Physical-layer security (PLS) is an effective approach to protecting user signals without requiring encryption, making it particularly valuable for rapidly deployed and temporary wireless systems such as unmanned aerial vehicle (UAV)-based networks. This paper focuses on maximizing the average secrecy sum rate (ASSR) in a UAV-based two-way relay system assisted by friendly jamming (FJ) signals from an additional UAV. The joint optimization of UAV transmit power, flight trajectory, and FJ signals for ASSR enhancement is investigated by formulating a tightly coupled problem, which is solved using the block coordinate descent method. Simulation results show that the proposed system achieves a 30% improvement in ASSR compared with systems without FJ signals and surpasses other benchmarks. Additionally, the adverse effects of FJ signals on legitimate users are thoroughly analyzed through the ASSR.