Can I take 400mg (2 perles) of Benzonatate at once, since taking the one perle of 200 mg isn't relieving my cough?

Not recommended. You are unlikely to get any additional benefit from a higher dose, but you may get unpleasant side effects. Consider a different cough suppressant, such as dextromethorphan or honey. If these are inadequate, your doctor may prescribe another medication depending on the cause of your cough.
Cough. 2 peerless will not do you any harm. However I do not know if it will help your cough. If it doesn't then you need to see a doctor and get some other treatment .