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hi, Boudewijn

After my test, you could do these follow my steps as below:

Assume that this is my data model

Step1:

Create a date table and create a Year Month Number column for each data table

Year Month Number = YEAR ( 'Date'[Date] ) * 100 + MONTH ( 'Date'[Date] ) 

date table

left table

right table

Step2:

then create a measure

Measure = var maxdate=MAX('Date'[Year Month Number]) return
CALCULATE(SUM('LEFT'[Sales]),FILTER('LEFT',maxdate='LEFT'[Year Month Number]))+CALCULATE(SUM('RIGHT'[Sales]),FILTER('RIGHT',maxdate='RIGHT'[Year Month Number])
)

Step3:

Drag the date field form date table and the measure into chart visual

here is pbix file, please try it.

https://www.dropbox.com/s/ny6bnkqp92skvdq/undefined.pbix?dl=0

Best Regards,

Lin