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.