You can use the eBike Connect app to log on to another on-board computer. For this, go to the "My eBike" tab. There you can manage and add devices. You can then choose the Bosch eBike on-board computer you want to connect to. If the on-board computer is logged on via another user account, all data that has not yet been synchronised will be deleted.
You can also connect directly to the "external" Nyon using your account. On the Nyon, select "Settings" > Connections > Connect Smartphone" and follow the instructions.