Under weak grid conditions, grid-following (GFL) inverters have broadband oscillation risks, while grid-forming (GFM) inverters have good stability. To investigate the impact of introducing GFM control on the stability of GFL inverters under weak grids, small signal impedance modeling was first performed on GFL and GFM inverters based on a dual machine system. According to impedance stability criteria, the operating characteristics of GFL and GFM inverters under different grid strengths were compared and analyzed, and the reasons why GFM inverters are more suitable for operation under weak grids were explained. Stability analysis of a dual machine parallel system was conducted, and it was concluded that the introduction of GFM control can effectively enhance the stability of GFL inverters under weak grids. Finally, the correctness of the conclusion was verified through MATLAB/Simulink and HIL simulations.