GO:0008750

Catalysis of the reaction: NADPH + H+ + NAD+ = NADP+ + NADH + H+. The reaction is A-specific (i.e. the pro-R hydrogen is transferred from the 4-position of reduced nicotinamide cofactor) with respect to NAD+ and B-specific (i.e. the pro-S hydrogen is transferred) with respect to NADP+.