If I understand what you're trying to do, I think there's a couple of solutions, here's what I would do:

Show the utility drawer so you can see your masters well in both documents. Drag the 'source' master to the destination document.

Now, drag the 'source' canvas to the destination document's canvas well in the utility drawer.

If all goes well, you should have an exact copy, keeping the master relationship intact. If your second document already had a master with the same name, you might need to re-apply the dragged master to the canvas.

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Alternatively, you could copy and paste the contents of the original master to a master in your new document. then repeat with the orginal canvas, pasting to a canvas in your new doc.

hope that helps,

schwa