If you have both projects open, you can just drag and drop.
The other trick is that when you export one or more displays, as well as an xml file for each display, you also get an extra file - the batch import file. This batch import file just says "import file xxx, import file yyy, import file zzz" for each of your displays - so if you've only exported the one display, it'll literally be a one line file saying "import file xxx". Then, when you go to import, choose to do a batch import rather than a single file import, and pick the batch file instead of the display file. Whether the batch file is for one display or one hundred displays, it'll create new displays for everything, instead of asking you to add it to an existing one.