Lexis

You can use a quick shape for this, place and adjust, select>selection from layer and delete

You can add to the selection afterwards using the add section of the context tool bar. The below post from MEB might help you with this further. If the object you are trying to select isn't circular I would recommend using the free hand selection tool.
 

Hi Lexis,
As far as I'm aware there is no way to do this in Affinity I'm afraid. I recommend making a post regarding this in our Feature Requests section if you would like to see it added to the app
Thanks
C

Another way would be to select one of the embedded documents with the Move Tool, click on the Replace Document button and replace the document with exactly the same document from your hard drive.
You then have 2 ArtBoards with the same embedded document but they are not linked in any way.
So when you make changes to one of them (on the embedded document) the other is not affected

The best way to do this is to avoid embedding anything in a new file that you don't want connected because it is an embedded file. If you still have the original Qube1 file, open it (or a duplicate of it if you want to keep an unaltered version of the original), & if necessary convert it to an artboard using the Artboard tool. Then you can duplicate the artboard with the Qube1 content without creating any embedded document connections between the two artboards.
If you do not still have the original Qube1 file, in the file with the embedded document, double-click on the "1" (or anything else in a artboard with an embedded document in it) to open that document in a new window. It will have the title "<embedded>" & can't be saved, but you can select all its layers, copy them to the clipboard, & create a new Qube1 file using File > New from Clipboard. EDIT: Lee beat me to it! 
BTW, Symboles are a way to create multiple instances of the same object in a document. Embedded documents are sort of the same thing, except they are entire document files.

Once you have your Artboards setup you can use the tools available such as Text Tool and Place Image (File menu) to insert content onto each Artboard, these can then be edited individually. An Embedded Document doesn't have an option to be unlinked, another option is to open the embedded file double-clicking on it (it will open in a new tab) copy the contents of it to the clipboard and paste them in the original document. Then duplicate those as needed,  they will be independent from each other.