Can you double up on prenatals?

NO-Think of the baby. After a baby is born, would a parent give it a double dose of any medication, including vitamins? We doubt it. Physicians prescribe them based on the benefits vs risk for mom+fetus and the dosage determined by the best information available. If there is doubt about the drug or dose, the prescribing physician should be consulted. All medications, otc and herbal products should be discussed with ob.
More is not better. Many vitamins are toxic when taken in excess. Even under normal conditions, vitamin toxicity is very dangerous, much more so when you are pregnant. Better to just use the recommended doses as directed by your doctor.
Not necessary. It is not necessary, nor is there any health benefit from doubling up the prenatal vitamins. In fact, some early studies had shown that taking excess Folic Acid (>1 mg/day) may be detrimental to the fetus. It really isn't necessary to double up.
Why? What's the reason in doing so? It gives no health benefit for any reason. You are risking a chance of having more adverse GI effects from it. Also, if you're taking it not being pregnant, a recently published finnish study showed a 4% increase in deaths for those taking multivitamins for no "good reason".