Post #9 in this thread already describes how you would go about adding more sets.
Although that post refers to identifying the target content controls by index number, a better way might be to reference them via their own titles, using code like:
Code:
ActiveDocument.SelectContentControlsByTitle("ClientDetails").Item(1).Range.Text = StrDetails
As for the error message, I don't know what that's about - the code works fine with multiple content controls in my testing.