Custom Double Heading for Matrix

Hi All,

I am building a report and my users are looking for a 2nd header for a better visual display.
Below is a sample what I am trying to achieve:

I am looking for the very top headers to encase the categories. (Example Header 2 -> 'Expected Electricity Use')

Key notes:
1) Matrix format is currently preferred as I need to show subtotals/grand totals per row

2) The 'Header types' do not have a consistent number of columns and all include different categories. (Main requirement that prevented other solutions from working)
3) The actual report is longer horizontally, requiring the horizontal scroll bar. A text box workaround does not work.

I can provide more details if anything is unclear.

Thanks a bunch for your time.