Slicer pointing towards multiple fields

There's a few steps involved but it is certainly doable.

Rather than using a disconnected table you could use a calculation group. Create an inactive relationship from DimUser[User] to FactDummy[User2], then create a calculation group with 3 items.

Both = 
VAR User1 = SELECTEDMEASURE()
VAR User2 = CALCULATE(
    SELECTEDMEASURE(),
    USERELATIONSHIP( DimUser[User], FactDummy[User2])
)
VAR Result = User1 + User2
RETURN Result

User1 = SELECTEDMEASURE()

User2 = 
CALCULATE(
    SELECTEDMEASURE(),
    USERELATIONSHIP( DimUser[User], FactDummy[User2])
)

Add that as a single-select slicer.

You need a measure to be able to filter on, you can use

# Rows = COUNTROWS( FactDummy )

Add this to your table visual, and you also need to add DimUser[User] to the table visual. Use the filter pane to set a filter on the [# Rows] measure to only show when the value is >= 1.

If you don't want them to actually display in the table visual, turn off text wrap on the values and column headers then set the width of the column to 0.

You could also hide the [# Rows] column in the visual by going in to the visual calculations and using the hide icon, though you don't appear to be able to do that for the DimUser[User] column.

See the attached PBIX for a working example.