Parallel virtual synchronous generator (VSG) systems are emerging as an attractive solution for inertia support and frequency stability enhancement of the power grid. However, the dynamic frequency characteristic is more complicated due to the coupled interaction of line impedance, virtual inertia, and damping coefficient in parallel VSG systems. The problems for frequency stability, dynamic performance, and frequency/power oscillation have raised much more attention. In this article, a systematic analysis of the influence of virtual inertia and damping coefficient on the dynamic frequency characteristic among parallel VSG systems was presented, and a quantitative assessment for dynamic frequency performance based on the 2-norm was proposed. Furthermore, a coordinated control for virtual inertia and damping was proposed to improve frequency stability and achieve a better dynamic performance of parallel VSG systems. The effectiveness and superiority of the proposed method were verified by simulation and experimental results.