I'm not sure if this will work for your true setup, but maybe it can give ideas.
I unioned the data to itself, which generated a new field called [Table Name]
with two values: Orders and Orders$.
This I used to make a new Month field:
IF [Table Name]="Orders" THEN DATETRUNC('month',[Order Date])
ELSE DATEADD('month',1,DATETRUNC('month',[Order Date]))
So the data coming from the Orders$ copy will be the previous month's data.
This new MonthLink calc was used in the area graph, but that graph was
filtered to just TableName=Orders to give the true undoubled results.
This seemed to allow the tooltip to work pulling both months.
Please see workbook v18.1 attached in the Forum Thread:
288800tooltip2.twbx 1.2 MB