There are countless ways to achieve this. The easiest is probably if you load both tables in the script (and ensure that CustomerId is the only common field) then QlikView will automatically join the tables on this field. Then any chart with CustomerId as a dimension and maybe two expressions - one that counts a field in table1 and one that counts a field in table2. Any row with a count in both expressions gives you the answer.
As I say though, this is a very broad question with many many solutions.
Hope this a helps.