If the Muay Thai training is pretty demanding (which it should be), then I's recommend training your BJJ on different days - at least as a beginner. The reason is that if you're exhausted by the MT training your brain won't be able to fully concentrate anymore. This is really bad for learning new stuff. You won't have full focus and won't execute the techniques properly. That way you will naturalise sloppy technique, because your body and brain just trained sloppy technique. It won't correct it by itself, but memorise what you did and that's sloppy technique. For a beginner it is very important to learn clean and propper technique to get a good technical base.
Later when you're more advanced you can do multiple demanding lessons consecutively, as it's more about using your technique in pratice than learning completely new stuff.