Understanding Multiple Fact Tables

I'm not sure if this is the "best way" and may be considered bad practice.

However I normally make use of TREATAS in these cases to move the list of customers from one fact to another.

Something like:

CALCULATE(

[Your Measure],
TREATAS(

    VALUES( Campaign[CustomerId] ),

    Customer[CustomerId]
))

If anyone has a better suggestion would be interested.

Here is one way. First the sample model:

 With a SUM measure for sales, create the following:

Sales By Campaign & Customer =
VAR StartDate =
    CALCULATE (
        MIN ( 'Campaign Table'[Campaign Date] ),
        ALLEXCEPT ( 'Campaign Table', 'Campaign Table'[Campaign] )
    )
VAR EndD =
    CALCULATE (
        MAX ( 'Campaign Table'[Campaign Date] ),
        ALLEXCEPT ( 'Campaign Table', 'Campaign Table'[Campaign] )
    )
RETURN
    CALCULATE (
        [Sum Sales],
        CROSSFILTER ( 'Campaign Table'[Customer], 'Customer Table'[Customer Name], BOTH ),
        FILTER (
            ALL ( 'Sales Table'[Sales Date] ),
            'Sales Table'[Sales Date] >= StartDate
                && 'Sales Table'[Sales Date] <= EndD
        )
    )

 to get:

I've attached the sample PBIX file