Bupropion works very fast by itself, where it works at all. Most likely people who benefit from bupropion get no benefit from the DXM, and vice versa. So maybe

Bupropion works very fast by itself, where it works at all. Most likely people who benefit from bupropion get no benefit from the DXM, and vice versa. So maybe this is supposed to be a scattershot to try two things that might work. If it comes back after a week without, it was the BUP that worked, otherwise the DXM.