What an intriguing question. I'm not a doctor and can't speak to the chemical nuances between the drugs, but one basic reason that doctors may not prescribe medications in such a way is that they have not been tested nor FDA approved for use in such a fashion. These drugs (and others) reached the market after going through trials with thousands of people taking them in a specific way, and even then there have been situations when an adverse effect was not known until the drug was taken by a wider population. In some cases, doctors do prescribe drugs for off-label uses (say, using a bisphosphonate for a premenopausal woman with osteoporosis when it has only been approved for use in postmenopausal women). However, for your safety. many physicians would hesistate to try out an experiment such as alternating medications on an individual patient.