If you lack an existing secure channel or access to a cable as suggested by Raiden, you can create a secure channel by creating a temporary chatmail account on your desktop, scanning the contact QR code with your mobile, and passing the authorization string this way to your new temporary account. You can delete your temporary account later if you no longer need it.
However this is maybe not an obvious workaround.
Someone else already suggested inverting the flow for adding a second device, or allowing it in both directions, as for contacts: