Yes, both stacked up and piled up mean the same thing!

"Stacked up" also means measured up or compared. "How does the new iPhone stack up to other smart phones?" "The new restaurant is not all it was stacked up to be." In this context, it is NOT interchangeable with piled up.

Hi, Mohamed: As Karlee suggested, these can be synonymous phrasal verbs. However, “stack up” can have another meaning: “to be good enough or equally good.” For example: (1) ”This model stacks up well when compared with other cameras. [=the camera is as good as or better than others]” There is also to “stack up against,” which is: “to compare (something or someone) with others of the same kind.” Definitions and examples were taken from the site below. Check out the entries for more examples: http://learnersdictionary.com/definition/stack http://learnersdictionary.com/definition/pile Thank you, Oliver