The integration of large-scale renewable energy sources displaces conventional generator commitments, leading to prominent low-inertia issues in receiving-end AC systems. To improve the frequency stability of interconnected power systems with high shares of renewables and power electronics, focusing on the back-to-back (BTB) VSC-HVDC system, this paper proposes a bilateral frequency support strategy for BTB VSC-HVDC with virtual synchronous generator-based grid-forming control. The grid-forming control method is employed in both terminals of VSC stations, enabling symmetric frequency support for two interconnected AC systems. By adjusting a specified coefficient, the degree of power support provided by BTB VSC-HVDC can be flexibly modified, achieving dynamic sharing of frequency regulation resources between interconnected AC systems. Finally, small-signal analysis and simulations in PSCAD are conducted to investigate the influence of control parameters and validate the effectiveness of the proposed strategy in enhancing system frequency stability.