This would be a two step search

1.       Go to Search Contacts in the menu bar. If the Selection Mode section is visible make sure that it is set to New Selection. Search for Contact Type equals sendtorep and Campaign equals [Action Alert 1]. This will find all the people who have taken action on [Action Alert 1] (whichever alert you are searching for at the moment)

2.       Go back to Search à Contacts in the menu bar. Set Selection Mode to Remove and then search for Contact Type equals sendtorep and Campaign equals [Action Alert 2]. This will remove all the people who have responded to [Action Alert 2] from your selection.

The list that remains should be the people who have taken action on [Action Alert 1] but not on [Action Alert 2], where Action Alerts 1 and 2 are Action alerts within your system.