Although I don't have an example specific to Spend Across Multiple Departments, we use a d-form to track Overall Savings from certain categories. I've attached a screenshot.
The d-form configuration could take some possible alternative structures. If you always divide by the number of departments, your fields might look like this:
Spend Number of Departments Spend per Department
[A calculated field]
If you divide unevenly, your fields might look like this:
Department Spend (or) Spend Percentage and then Spend (Calculated)
Now, in your reporting, you can list the overall spend from the profile field and then the department spend from the d-form, which ever way you approach it.
Savings Form.bmp 1.6 MB