wait what? why are you talking in riddles?
what do NMDA antagonism and liver enzyme 2d6 have in common? i dont understand. the 2d6 enzyme determines how much DXM is converted into DXO, which is a powerful NMDA antagonist. DXM itself is a weak NMDA antagonist, and DXO plays a major part in the pharmacological effects of DXM. methadone, a weak NMDA antaognist as well, is processed through the 2d6 enzyme, along with other enzymes of the liver into other metabolites. if DXM and methadone are taken together, the methadone will inhibit "some" amount of dxm converted to dxo, and dxm will inihibit "some" amount of methadone metabolized into other substances. how much the 2d6 enzyme will be inihibited is based on a multitude of different variables, so it is hard to say how much DXO will be active, etc...
overall, the amount of methadone and DXM/DXO in the blood stream does not affect how the drugs will act inside the CNS. so no, taking DXM and methadone together is not "useless" per se
i am still kind of confused on what the fuck you are asking, i dont even know if you are asking in english lol. but i tried to understand what you were asking and answer your question to the best of my ability