In this paper, the performance of nonlinear wireless power transfer (WPT) enabled by multiple radio-frequency (RF) sources is characterized. The harvested power is modelled as a function of the RF powers received from multiple RF sources. Following the characterization, we prove the joint convexity of the harvested power under different sufficient conditions. In particular, based on these conditions, the joint convexity of the harvested power with respect to the reciprocals of the received powers is proved. Moreover, by providing an example design, we illustrate the benefits of exploiting the joint convexity for cooperative WPT enhancement.