I think the rules specifically mention phase actions (performing non-response special abilities, playing non-response event cards)--basically actions using a bold phase name. You choose one and complete it before you can perform another, so as mentioned copies of
Morgul Gates can't be combined.
Triggers include actions beginning with "When", "While", "Each time", "At the start of", etc. When the described situation happens, all cards that meet the situation trigger. An example of this would be each active copy of
Orc Banner triggering when a companion loses a skirmish. Though technically you still aren't combining them since they're resolved one at a time.