NPDE for parent and metabolite data

Dear group, Yet another question to the group. I am working on a model for drug molecule with parent-metabolite data. I am requesting NPDE in NONMEM 7.2 and plotting NPDE overlaid on standard normal distribution to see the goodness-of-fit. Now, do I need to plot different plots for parent and metabolite for such plots as with other diagnostic plots? Also, how the decorrelation is handled when two analytes within an individual are given? Regards, Ayyappa