I recommend a relationship on the IDs between the two tables.

Then on the Table_start Table, create a calculated column:

date diff = 
VAR startDate = Table_start[startDate]
VAR endDate = RELATED(Table_end[endDate])

RETURN DATEDIFF(startDate, endDate, DAY)

Create a measure to get the average:

average days = AVERAGE(Table_start[date diff])

Result: