Hello from JustAnswer.
No, not really, although doctors will typically only prescribe drugs as they have been shown to work.
Once a drug has been approved by the FDA for specific indications, doctors are not limited to those indications. If there is clinical evidence to support using that drug for different conditions or in a different manner, then doctors can prescribe medicines differently than the FDA approved indications.
If a doctor decides to use a drug for a reason not approved by the FDA or in a manner different than approved, there is some degree of medicolegal risk, but as long as there is other evidence in the literature to support the use, the doctor will usually not have problems from a legal perspective.
Therefore, doctors are not limited to only prescribing the two drugs together, but the practical issue is that doctors will typically only prescribe the drug as approved until there is other evidence to support a different method of use.
If I can provide any further information, please let me know.