As of 2019, the FAA and EASA require all airline pilots to complete stall and recovery training as integral part of their training. To mitigate risks, this training takes place in ground-based simulators. To enable this, realistic models of aircraft behaviour in the stall regime need to be developed. In this paper, a newaerodynamic stall modelling methodology is proposed that combines classical aerodynamic model identification techniques with a novel adaptation of Kirchhoffs theory of flow separation that considers flow separation over both wings separately. This new model is called the 2X model, as it contains 2 independent flow separation variables, i.e. one for each wing. The model parameters are estimated based on flight experiments in the stall regime conducted with a Cessna Citation II laboratory aircraft operated by the TU-Delft. The developed model for the first time allows accurate prediction of lateral-directional dynamics encountered during stall such as e.g. wing-dip. In addition, it was found that the 2X model also improved predictions of longitudinal stall dynamics leading to a new extended envelope aerodynamic model for the Cessna Citation II that now also includes stall entry and post-stall aerodynamics. @en