A 45-year-old female with major depressive disorder presents after failing adequate trials of sertraline and venlafaxine over the past year. She reports persistent low mood, anhedonia, fatigue, and difficulty concentrating. She has no history of seizure disorder, eating disorder, or current use of MAOIs. The PMHNP discusses dextromethorphan-bupropion (Auvelity) as a treatment option. The patient asks how this medication differs from the antidepressants she has already tried. Which of the following best explains the pharmacologic rationale for this combination?