Filter with two Values

Hello,

I have a problem in DAX and can not find a solution after days of searching. Perhaps someone can give me a hint? 
I have the current MEASURE Definition: 

CostUSER = 
VAR currentuser = [SELECTUSER] -- comes from other measure 
VAR varD = 
        SUMMARIZE (
            FILTER(USERDepartment,USERDepartment[USERID]=currentuser),
            USERDepartment[Department] )
    )
VAR calcCost = sumx(FILTER(RegionCost,related(DepartmentRegion[Department]) IN varD),RegionCost[Cost])
RETURN IF(ISBLANK(varD),BLANK(),calcCost)

How it possible to FILTER related(DepartmentRegion[Department]) with two Values ?  
The user in USERDepartment can be assigned two 2 Departments. If a choose with a slicer one department everything works but with two Departments it says that one value instead of two was expected. 
I try different things like TREATES, but with no success. 

The Tables

One hint we be so great! Thank you! 🙂