In high doses, mixing CNS stimulants and respiratory depressants can lead to serious complications such as a coma or death.
IME, low-moderate doses of the two drugs are not worth mixing... If I am trying to get fucked up and nod off of an opiate (which would never happen with codeine anyways), amphetamine just seems to get in the way of opiated euphoria. On the other hand, if you have classes, work, whatever, it might be beneficial to stay awake while on your codeine. I guess what I'm trying to suggest is save your drugs and do them individually.
But alas, someone who is naive to opiates will probably post that they love mixing the two, so whatever. Do what you want. Might as well try the mixture just to try it, that is always the way I justify doing stupid shit like mixing uppers and downers.