Integrating renewable energy into the power grid through self-synchronizing voltage source inverters (SSVSIs) is beneficial for frequency stability. However, factors such as line impedance and control parameters make the frequency response characteristics of multi-SSVSI grid-connected systems complex, and there is a lack of research on the coordinated frequency support control mechanisms for multiple SSVSIs. To address this issue, a frequency response model of a multi-SSVSI grid-connected system is first established, and key factors affecting frequency response characteristics are analyzed. Subsequently, a coordinated control method for frequency support for multi-SSVSI grid- connected systems is proposed. The proposed method enhances the overall system response speed through power compensation control. It further distinguishes between the inertia response stage and the recovery stage of frequency response, and adaptively adjusts virtual inertia and its allocation in a differentiated manner to achieve dynamic coordination among multiple SSVSIs. As a result, the rate of change of frequency and the maximum frequency deviation under load disturbances are reduced, and the system frequency recovery speed is improved. Furthermore, design principles for key parameters are presented. Finally, simulations and experiments validate the correctness of the analysis and the effectiveness of the proposed control method.