[Objective] Grid-forming ultra-high voltage （UHV） flexible DC converter stations are a crucial solution for supporting the large-scale transmission of high-share renewable energy. However， their multi-valve group series-parallel structure under grid-forming control is prone to issues such as uneven DC voltage distribution and energy coupling. This paper aims to uncover the energy coupling mechanism in dual-valve group systems and proposes a corresponding control strategy to achieve energy decoupling among valve groups and internal capacitor energy balance. [Methods] First， an energy interaction mathematical model of the dual-valve group system is established to theoretically analyze the generation mechanism of energy coupling. Subsequently， a master-slave grid-forming control strategy for modular multilevel converter （MMC） with dual valve groups， based on submodule energy balancing theory， is proposed to suppress inter-valve group energy interaction and achieve decoupling control. Finally， the correctness of the theoretical analysis and the effectiveness of the proposed control strategy are verified through an electromagnetic transient simulation model of a sending-end grid-forming UHV MMC with multiple valve groups built on the PSCAD/EMTDC platform. [Results] The proposed control strategy effectively suppresses the energy interaction between the dual valve groups， achieving energy decoupling between them. Simultaneously， it balances the submodule capacitor energy distribution within the converter valves and between the valve groups while maintaining stable power transmission. [Conclusions] Through mathematical modeling and a master-slave grid-forming control strategy， the issue of energy coupling during the operation of grid-forming UHV MMC with dual valve groups has been effectively addressed. This approach not only achieves energy decoupling and capacitor energy balancing between valve groups but also provides a theoretical foundation and a control solution for the stable operation of multi-valve group flexible DC systems in practical engineering applications.