This paper first designs a holistic coordination framework between transmission system operator (TSO) and distribution system operators (DSOs) to modernize traditional top-to-down (from transmission system to distribution systems) power flexibility provision mechanism to a bi-directional power flexibility provision mechanism between TSO and DSOs. More specifically, it empowers TSO and DSOs to exchange both active and reactive power flexibility without having to reveal their confidential grids data. Above all, it allows TSO to take advantage of the potential active and reactive power flexibility of the proliferating number of distributed energy resources (DERs) installed in distribution systems. Secondly, it develops a linearized power flow model for transmission networks. Leveraging the designed framework along with the developed linearized power flow model, it finally offers a two-stage linear stochastic optimization method to help TSO optimally book its required active and reactive power flexibility from both power plants and distribution systems. In particular, it considers constraints and active/reactive power losses of the transmission network. The performance of the proposed framework is evaluated considering a real-world transmission network, i.e. the Swiss transmission network.