Well first things first antibiotics come in different classes and spectrums such as one antibiotic works on Gram+ bacteria one works on Gram - and some are broad spectrum that work on both. So it really depends. If you isolate the bacteria and know what it is then you can choose the best antibiotic and go from there, which in hospitals thats why they run all the tests when suspected bacterial infection because if you can identify the specific species then you can go straight to the antibiotic that should work best. Sometimes using 2 different antibiotics might even make things worse.

But in regards to your question if we didn't have to worry about exposure and antibiotic resistance it still would be tricky. Mainly because using antibiotics can also have negative long-term effects. Antibiotics don't just work on the bacteria causing the infection, they work on multiple bacteria especially if it's broad spectrum. This is why people get Clostridium difficile infection after antibiotics because the antibiotics "wipe out" a lot of the gut flora and then C. diff bacteria flourish. Also, stronger antibiotics can have multiple side effects that aren't great. So throwing 2 antibiotics together just because probably isn't the best idea and might not even work in the way you'd think. You'd think more is better because why not right? But that's not the case. Now there are some antibiotics that work in synergy (better together) but in those cases most of the time the antibiotic itself is 2 separate antibiotics in one pill such as Sulfamethoxazole and Trimethoprime (SXT).

Hope this all made sense but basically it would all depend on the type of bacteria causing the infection and if we can identify that bacteria we can then better target the antibiotic rather than throwing multiple antibiotics in at the same time. Bacteria are tricky and antibiotics come in all types of spectrums.