I find the 'Thumbnail Drag' feature to be great for this. Basically, open your two documents, and go to View > Thumbnails in both documents. Adjust the size of the windows so you can see pages in both documents.
Now you can simply click and drag pages from one document to the other. Quark also allows you to select and drag multiple pages at once.
Just be sure that both documents are the same size and it should work. I should also point out that it doesn't delete the page from the first document, it only makes a copy.