It is possible to do this as you require without using Ring Group.
There are two ways to do it:
1.- If you use PJSIP technology, you configure in Max Contacts the number of devices you want to register at the same time. In this case you only use one account and all your devices will be registered to this account.
2.- With the Multi-Device option, in this case it is possible to mix technologies such as SIP, PJSIP, etc.
The way to do it is to create the first extension, for example 1001, then you are going to edit it and in the Devices section you select New in the Device field and create the second device, and thus you can create the devices you want associated with the same extension.