This paper studies the power flow control of two LCC-HVDCs interconnected through DC-DC Autotransformer. The interconnecting scheme and its potential benefits are firstly summarized. Natural instability of using conventional LCC controller in the interconnected system is identified. To solve the instability issue, local power flow controller and centralized power flow controller are designed. Simulations on interconnecting a ±500kV, 3000MW LCC-HVDC and a ±800kV, 7200MW LCC-HVDC through a ±500kV/±800kV, 1000MW DC-DC Autotransformer verify efficacy of the proposed controllers. The proposed power flow controllers enable stable operating of the system with minor modifications of the conventional LCC-HVDC controls.