Chiral switches are an interesting part of drug development as this can easily produce two separately marketed products. Chiral switches involve playing around with the stereoselectivity to create very similar yet distinct enantiomers, thereby potentially improving efficacy and reduce adverse effects.
Here are some two-drug combinations which involve separating out a specific enantiomer from a racemic mixture:
1) Ofloxacin and levofloxacin: antibiotics which have different bacteria-killing spectrums
2) Omeprazole and esomeprazole: proton pump inhibitors to reduce acid secretion, where esomeprazole theoretically has improved effects
3) Zopiclone and eszopiclone: z-drug hypnotics for insomnia
4) Citalopram and escitalopram: selective serotonin reuptake inhibitors for depression, where escitalopram has a faster onset of action and less adverse effects