How can get the sum of any two months in Smart View or Financial Reporting?

Summary:

Our customer needs to select any two months from period dimension (consecutive or not), and have its addition as a result, in one column.

He can now select one month, quarter or period-to-date selection, but he needs to select for example september and october, and have sept+oct as a result.

There used to be a way to do something like that in Web Analysis, but we cannot find a way nowdays in EPM.

How can that be done in PBCS?

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable): EPBCS