I think I didn't explain correctly the situation.
When the page loads, the user only see the information of the first tab (the second one is hidden by display:none), so I send an impression with these products (products in first tab).
Only when the user clicks on the "tab2" link (and appears the second tab) is when I need to send the new impression (products in second tab).
I can use a comma-separated list of selectors:
#contentTab1 .contentProd h3, #contentTab2 .contentProd h3
But in that case I would send the impression of all products when the page loads, while I need to send the list of products of the second tab only when it is shown.
I guess I need to use a different extension to send the impressions for the second tab.