Hope this helps, but this layout need data duplication which I don't know your data connection allows or not.
Other simple approach is create two tables to be joined on dashboard.
From edit datasource ==> union
Lots of steps of calculation
make "date" as just monthly grain. not to allow users to expand "date"
to make this LOD works, need to change the filter to context.
Set sort not by alphabetical but by date
SampleMonthGroup_SM_10.5.twbx 127.2 KB