GrandTotals show the aggregated value of all cells above them and they are not designed to show two values. However there are two options that you can use:
1) Use CellTemplateSelector and apply different template for the GrandTotal. In this case you will have to calculate and set the second value.
2) Hide GrandTotal row and use two Calculated Items - one that will aggregate all values (similar to GrandTotal) and one for your Net value. This way you will have two rows, but the values will be calculated by RadPivotGrid
and there will be no need of manual calculation from your side. You can find more information about CalculatedItems in our online help
We've prepared a sample project, that demonstrates both of the above approaches. Please give it a try and inform us in case you have any problems or concerns.