Grid-connected systems with multiple self-synchronizing voltage source inverters (SSVSIs) (referred to as multi-SSVSI grid-connected systems for simplicity) are exposed to low-frequency oscillations (LFOs) and synchronous frequency resonance (SFR). However, the synergistic suppression of these two oscillation modes has not been achieved to date. Considering the dynamic characteristics of transmission circuits and power coupling characteristics, an improved power-frequency (P/ω) admittance model for multi-SSVSI grid-connected systems is established, reflecting the mechanism of LFOs and SFR, as well as the positive and negative effects of corresponding control parameters. In addition, an oscillation suppression method is proposed to enhance the system damping by adding virtual resistance control and transient virtual power feedback control, simultaneously suppressing LFOs and SFR. Unlike existing methods, the proposed method introduces virtual active power instead of real active power as the power feedback into the control loop, thus suppressing LFOs with a smaller transmission resistance and avoiding severe power coupling. Experimental results verify the effectiveness and superiority of the proposed method.