[proaralyst]

I'm not a biologist, but I'm guessing that having the concentrations of two antibiotics peak at the same time etc is much much trickier than having one molecule with two mechanisms of effect. Thus in actual (not perfect) use you end up with both effects active within a bacterium at one time, rather than potentially having some distance between the peaks.

From the article, it does look like they're smushing two antibiotic active sites into one molecule, so it seems a somewhat similar idea