Involved in the secondary bile acid metabolism. Catalyzes two subsequent reductions of the double bonds within the bile acid A/B rings of 3-oxochol-4,6-dien-24-oyl-CoA and 12alpha-hydroxy-3-oxochol-4,6-dien-24-oyl-CoA to yield 3-oxocholan-24-oyl-CoA and 12alpha-hydroxy-3-oxocholan-24-oyl-CoA, respectively. 3-oxocholan-24-oyl-CoA + NAD(+) = 3-oxochol-4-en-24-oyl-CoA + H(+) + NADH 3-oxochol-4-en-24-oyl-CoA + NAD(+) = 3-oxochol-4,6-dien-24-oyl-CoA + H(+) + NADH 12alpha-hydroxy-3-oxocholan-24-oyl-CoA + NAD(+) = 12alpha-hydroxy-3-oxochol-4-en-24-oyl-CoA + H(+) + NADH 12alpha-hydroxy-3-oxochol-4-en-24-oyl-CoA + NAD(+) = 12alpha-hydroxy-3-oxochola-4,6-dien-24-oyl-CoA + H(+) + NADH Lipid metabolism; bile acid degradation.