Why is Artemisinin Compound Therapy preferrable over Artemisinin therapy with regards to drug resistance? How does adding other anti-malarial drugs to artemisinin, to obtain a "combined" therapy result in reducing the frequency of emergence of resistance? > And it is given in the form of artemisinin combination therapy, combined with another anti-malarial drug, with the goal of reducing the frequency of emergence of resistance. (Cf source).

The idea is rather simple. When an antibiotic is used, it becomes a driver to select for a mutant that has resistance towards the antibiotic. Let say the probability for that happening is P1 (Lets say 1 in 10, 0.1).

When you use two antibiotics with different modes of attack, the probability of acquiring resistance to both is P1 x P2. (Let say P1 =0.1 and P2 = 0.15). So P1 x P2 = 0.1 x 0.15 = 0.015

Hence the probability of becoming resistant to both antibiotics at the same time is very much lower. And as the antibiotics are always used together, the time a bug needs to develop resistance to the therapy becomes longer. The duration which you can use the drug is increased.