If two bodies are in thermal equilibrium in one frame, will they be in thermal equilibrium in all frames?

Yes, if two bodies are in thermal equilibrium in one frame, they will be in thermal equilibrium in all the frames. The two bodies in thermal equilibrium will be having same temperature and temperature is independent of frame of reference. Due to change in frame, if there is any change in the temperature of one body, the heat will flow from hotter body to colder body and as a result same change will be reflected in the temperature of other body. Therefore, the two bodies will be in thermal equilibrium even if the frame changes.