Most labs test for sensitivities to one antibiotic only at a time.
But many patients end up taking more than one at a time anyways. This will tell you which is the best combination of 2 or 3 antibiotics to use. Yes, it is much more expensive, but often worth it, in a last ditch effort. It may also be done for a patient with drug allergies, to find out what will work for him amongst drugs that he's not allergic to. Antibiotic Synergism means the action of one antibiotic, combined with the action of a second antibiotic, produces a greater effect than the sum of their independant activities.