If one cholesterol-lowering drug is good, two must be better. That was the thinking behind Vytorin, a combination of Merck's Zocor and Schering-Plough's Zetia. Not so, it turns out. A study of 750 patients showed that while the combination did lower bad cholesterol, or LDL, it did little more to reduce the dangerous buildup of plaque than Zocor alone did which costs far less than the combination drug Vytorin.