First create the extension, once created you edit it and go to the Devices section and press Device and you will see that it shows you new, then there you add the second device. There are no limits for devices associated with an extension.
With the new technology PJSIP allows you multiple devices connected at the same time, in this case you only need to say how many contacts you want in Max Contacts. This way you don’t have to create more devices.
All devices ring at the same time and if one is busy the others ring if a new call comes in.
With regard to putting on hold, this is done on the device, not on the extension.