counting rows that are dependent from two different tables

Hi aake,

After going over the sample data provided above, I would suggest you to merge "IN" table and "OUT" table into a single table(assume it is called "Merged_IN_OUT") first, then you should be able to use the formula below to create the measure "WIP".

WIP =
    CALCULATE (
        COUNTROWS ( Merged_IN_OUT ),
        FILTER (
            Merged_IN_OUT,
            Merged_IN_OUT[Start] < MAX ( workdays[Workdate] )
                && Merged_IN_OUT[Shipment] < MAX ( workdays[Workdate] )
                && Merged_IN_OUT[LastStatus] = "Shipped"
        )
    )

Here is the sample pbix file for your reference.

In addition, for more details about how to use Merge Queries in Power BI Desktop, you can refer to the following articles:

POWER BI DESKTOP : MERGE QUERY OPTIONS

Shape and combine data in Power BI Desktop

Regards