We consider joint RF energy harvesting (EH) and transmission control at two transmitters over wireless multi-access channels. Each transmitter can harvest energy from surrounding RF environment and signals of nearby transmission (TX), and operates in either an EH mode or a TX mode. We design an online joint operation mode and transmission power control algorithm, aiming at maximizing the long-term time-averaged sum-rate. By transforming the stochastic optimization problem and leveraging Lyapunov technique, we develop an online algorithm that only depends on the current battery energy levels and fading conditions. The sum-rate and EH performance are studied through simulation, and we demonstrate the effectiveness of our proposed algorithm over the alternative greedy approach.