MDX isn't magic, you will need to do the groundwork in the structure of your cube first. My thought is that you will need to utilise a many to many join.

Have a read of the following first:

http://www.sqlbi.com/articles/many2many/

Then move on to this:

http://blog.crossjoin.co.uk/2014/09/12/counting-customers-who-have-bought-all-selected-products/

For fairness' sake, you could achieve what you need with a distinct count, but you will take a performance/manageability hit:

https://technet.microsoft.com/en-us/library/aa902637(v=sql.80).aspx

Have fun.