You can have multiple sub summary parts, then when you sort your report for a different sub summary, that's the info that will be shown.
In this example the ReportPicker is a global drop down field with an OnObjectModify script trigger attached to it. When you pick a different report, it sorts on the appropriate sub summary field.
Given that each report lists both records and sub totals from different tables, putting both on the same layout will be difficult if not impossible.
Ooops...missed the 2 different tables.....agh!
If it weren't for the "records must be editable" part, I'd suggest setting up a report table and importing the data from both tables into a common table and basing your layout on this report table. This takes time to "build" your report via import each time, but solves most of the display issues.
I would suggest that you take a close look at your two tables and ask your self if it might be possible to build in a third table that links to your Request and Cost tables by FeWoID. This can be a separate set of thee Tutorial: What are Table Occurrences? used just for this layout. You'd have one record for each individual request and cost record, but with no one record linked to both. You can then set up a list view layout with two main sub summary layout parts and sorting to separate your records into Request and Cost groups so that you can put fields from both Request and Cost occurrences on your layout. You may have to put two completely different sets of fields on your layout layered on top of each other, but in theory, this could be made to work.