One method would be to filter the data out that you do not want displayed leaving only the customers that fit the criteria. I am attaching a a samaple workbook. In the workbook I used a calculated field and an IF statement to create logic that will only return customers that both appear in the [Active customer in last 12] months and [Active customer in last 6] months. I then dragged my calculation [!Active Customer] to the filter shelf and set it to 1. Being that 1 is when the arguments are satisfied.
To see the calculation, right click and select 'Edit' on the calculation [Active Customer].
*Note after I created the calcuation I converted it to discrete.
Hope this example helps!