Or avoid taking them one at a time, to put it another way.
If a population of a type of bacteria is treated with antibiotic A, maybe an individual in the population happens to randomly have resistance. Then that individual reproduces, and maybe it escapes into the wild, and later on has to be treated with antibiotic B, but by this point in time it has formed a second large population, so there's a chance of another individual appearing in the new population which has resistance to both A and B. The longer the gap between the first and second treatment, the larger the population, the greater the chance.
So using the antibiotics one at a time is setting evolution up with nice easy incremental steps. Use both A and B together in the first place, and the odds are much better of the whole original population being wiped out, including any mutant that could resist one of either A or B.
I don't know if it's actually OK to mix antibiotics like this, I was just amused by the thought, because standard wisdom for avoiding resistant bugs is "go easy on antibiotics", yet maybe if you're going to use them at all it's most ethical to gobble down every kind in sight. If you're going to challenge the bacteria with a fence to jump over, make it a really high fence.