Implementing the logical OR on members from different hierarchies

If we need to slice the data by only the black color for products, we would put the Black member in the WHERE clause, like this:

WHERE 
   ( [Product].[Color].&[Black] ) 

In the Adventure Works DW 2016 database, by putting Reseller Order Quantity and Reseller Order Count on the columns, we would get this result:

Similarly, to get only the products whose size is XL, we can put the member XL in the slicer as:

WHERE 
   ( [Product].[Size Range].&[XL] ) 

What if we want to get the products whose size is XL in the same result set as the result set for black only?

Somehow, we need to combine the black member with the XL member. Simply by putting these two members together, it would not work. Putting two members from different hierarchies would form a tuple; a tuple implies the logical AND in MDX, not the logical OR.

On the other hand, MDX implies a logical OR. However, we cannot simply put the preceding two members together...