Is this a half bridge module where they are integrated in the same packaging somehow then? It sounds like you want to model cross-coupling between the two MOSFETs. In this case you can include the logic similar to the attached screenshot of the interaction between and IGBT and diode in a module where each device's junction is connected to a common thermal layer (e.g., heat sink or ambient in your case):
If you know the coupling impedance (Rth_Qd in my diagram) you can place it then in parallel to both MOSFET paths and use a Gyrator style logic to influence the heating from one to the other, and vice-versa. I hope this helps, but let me know if you have additional questions.