Userelationship

Anonymous , Both date are from the same table. So you do not need userelation for date diff.

In a new column = datediff(order[due Date], order[Shipped Date],DAY)

In a new measure= datediff(min(order[due Date]), Max(order[Shipped Date]),DAY) // Refer my blog for row context

Now use relation is which date you want to use when you display this. So on top of this formula, you can use one of the date.

Or can have two formula one for each date.