This is much easier for bacterial infections vs cancer -- chemotherapy is (excuse the non-medical phrase) very close to actual poison. Antibiotics are very well-tolerated, so taking two at once is something that could be done and is indeed deployed in serious infections or when there is high risk of one.

This - nearly all antibiotics target parts of the bacterial cell that humans don't have. Antifungals have the problem that many of their targets are shared between eukaryotic fungal cells and our own. Chemotherapy agents have a huge problem in that they are targeting cells that used to be our own. It's a race between toxic to cancer cells and toxic to the rest of you.

For some bacterial infections where resistance is a concern, such as TB, this is already the routine standard. See also anti-HIV medication cocktails.