Gruntdoc wonders about why a particular combination therapy isn't available yet. Skin infections with methacillin-resistant staphylococcus aureus (MRSA), which I hope I never come any closer to experiencing, are treated with one of several antibiotic combinations, but they're all administered as separate drugs.
The answer is what you might suspect: the FDA would want clinical trials of the single-dose combination, just to make sure that things work the way that they're supposed to. Any company developing the combo would have to recoup those costs, not to mention the costs of then beating the drum for the idea that the new combination is a better idea. But the antibiotics in question are generics, which means that there could be some real cost-containment issues over the use of a more expensive combination.
But we have a rather close example at hand: the recently approve BiDil. (Here's the package insert, in PDF format.) That's a combination of two generics, too, which (famously) shows far better effects in the black population than it did in general clinical trials. Nitromed, the developer of the therapy, had to run some pretty reasonable-sized ones, and they spent a lot of money in the process.
They started by establishing that the blood levels of the two drugs were reasonable when given in combination, and went on to a group of 186 male patients. That trial (with 273 in the placebo group) didn't show a benefit, but hinted at one in the black subjects. The company also ran an 804-patient trial against enalapril, and saw the same trend, which led to the definitive 18-month trial in 518 black patients (with a roughly equal number in the placebo arm.) Keep in mind, this is all for two drugs whose individual efficacy was well-studied.
Note added after original post: Nitromed was after something more than the individual efficacy of each drug. Their hypothesis was that the combination would make the blood-pressure-lowering effect much more pronounced, and that this would translate into clinical benefit as seen in eventual mortality. Why this only seems to be the case in the black population is a head-scratcher. The situation for combination antibiotics would be simpler. So. . .
A combination antibiotic trial wouldn't be as long, or as expensive. But it wouldn't be negligible, either, and it's likely that some companies have run the numbers and decided that the investment would be unlikely to pay off.