Cashback apps track a purchase through an affiliate link and pay you back a percentage afterward, but running two cashback apps on the same purchase usually doesn't double your payout, it often cancels one of them out entirely.
The reason: most cashback apps work by being the 'last click' before checkout, the affiliate network only credits whichever app's link was clicked most recently. If you activate a second cashback app after the first, you typically overwrite the first app's tracking rather than adding to it.
The one combination that reliably does stack: a cashback browser extension plus a cashback credit card. Since the card's cashback is tracked by the card network based on the merchant category, not by an affiliate link, it doesn't conflict with whatever affiliate-based cashback app you used to get to the site.
A common mistake for beginners is activating a cashback app, then also clicking through a coupon site's affiliate link before checkout, that second click usually breaks the first app's tracking, and no cashback registers on either.
Simple rule for stacking correctly: pick exactly one cashback app or affiliate coupon site per purchase, and pay with a cashback rewards card. That combination reliably compounds; adding a second app or site on top usually doesn't, and can actually cost you the first app's payout.