It is up to the doctor. I'm not sure why you would take both verapamil and diltiazem since they are both CCB's. I'm going to assume the doctor is prescribing this and you're not just going to find the med and add it yourself. Ask the doctor about how the medications work and why you need both.
It really depends on what you need. If this has been prescribed for you, and you are uncertain about it, then you can always ask your doctor for more clarification. It really just all depends on your individual circumstances.