If you’re working on an iPad, use Split Screen to open another view of Prêt-à-Template. Once they’re side by side, open both files - the one you’re working on and the one you want to copy an element.
With the Scissor tool, select the area you want to copy.
Press the selection until you see the Scissor icon, then drag it to the other canvas.
And you’re done!
Watch this video if you need help understanding the Scissor tool.
You can also easily copy layers from one project to the other.