You can try to concatenate your fields so it would be something like the following:
SALES & FORECAST
IFNULL([SALES],0) + IFNULL([FORECAST],0)
This is assuming that in your data there is no week that has both Sales AND Forecast. You would want to wrap this in the IFNULL since you can't add a Null and a value together.
You could use the following calculation to display the Sales or Forecast value based on what week we are currently in:
IF DATETRUNC('week',[DateField]) <= DATETRUNC('week',TODAY())
This will be dynamic so will reference the Forecast value until we are in that specific week of the year.
Hope this helps!