Synergy
When two supplements produce a greater combined effect than either would alone.
Synergy means the whole is greater than the sum of its parts. Two supplements with complementary mechanisms can reinforce each other so the pairing outperforms either one taken on its own.
A classic example is caffeine with L-theanine: theanine smooths out the jittery edge of caffeine while preserving the alertness, giving calmer focus than caffeine alone.