Chlorox and ammonia are effective cleaners on their own and life threatening if mixed.

Sugar and acidity go together well but can be unpalatable on their own at the levels that theyre used. i.e 45 grams sugar / can would gag you without that acidity. the acidity on its own would not taste great.

Some spices synergize with other spices but arent great by themselves.

fat by itself: gag, bread by itself: boring, bread + fat: tasty

Carbon Dioxide is poisonous by itself but in small amounts it can stimulate breathing. Pure Oxygen can cause nerve damage over long periods of time but is necessary for survival. Nitrogen isnt poisonous but it cant support life on its own either. The three mixed together in the right proportions is breathable over the long term (air)

Penacillin type antibiotics dont work on resistant strains on their own and drugs that inhibit the enzymes that bacteria use to neutralize those antibiotics wont do anything on theur own but together, they can kill bacteria that used to be vulnerable to antibiotics but developed resistance to them.

Alloys are examples of materials that are greater than the sum of their parts.

Sulfur, charcoal and saltpeter are relatively unremarkable. Sulfur and charcoal burn but not particularly violently. Saltpeter doesnt do much unless it is paired with fuel. Together they make gunpowder.