How to find Enchantments or Artifacts that have multiple effects?
As a starting point, the following Gatherer search finds all of the cards with multiple triggered abilities that have the exact same trigger condition: text=m/(\b(When(ever)?|At)\b[^,]+),.+?\1,/. This uses Gatherer's regular expression search functionality to find cards with "When...", "Whenever..." or "At..." matching up to the first comma, and then the exact same text later on the card. This has a couple of false-positive matches of cards with commas in their name (like Brimaz, King of Oreskos). It also matches cards with multiple triggered abilities with the same trigger condition and different "intervening if" clauses (like Duergar Hedge-Mage).

It's a lot more complicated to find cards like Waste Not with trigger conditions that are similar but not identical, because it is unclear how that "similarity" would be rigorously defined in general.