You need to create a set of the two members to get the intersect and then sum it. Here's an example on adventure works for bike sales in the UK:

WITH MEMBER [UK Bikes]

AS

SUM(

{([Product].[Category].&[1],[Geography].[Country].&[United Kingdom])}

,[Measures].[Reseller Sales Amount])

SELECT

{[UK Bikes]} ON 0

FROM

[Adventure Works]

Hope that helps 🙂