For the 2 column nested sort BOTH in descending order scenario you describe, you can create a calculated field: [Sales Volume]*10000000 + [Days in Inventory] and sort descending on that calculated field without placing it in the view.
You didn't need to create a new thread, you should have attached it into your original thread.
I don't understand why you need to sort like this - in the example you've shown, sorting by sale volume would be exactly the same as sorting by sale volume first then by days inventory. The only case where you would see it change is if you had two products with the same sale volume.
Perhaps you can tell me how you would expect it to sort based on the sample data?