To put this in real basic terms.....
Any phone can register as an extension. The phone has settings for this. It needs the ip of the pbx, the port to register on, the id and password to use (and some nice to haves such as a display name for the phone but not needed for the registration).
All 3cx wants from the phone is to make sure the id and password authenticate and then it wants to know the ip and port of the phone to send the invites (this is all sent in the registration message from the phone).
To 3cx an ip is an ip, internal or external - it neither knows or cares.
So you can have 20 phones of different models as Leejor says, no need to clone, and as long as you point them to 3cx pbx and give the same user name and password for the extension they will all register. And will all ring when a call is for the extension.
So dont get confused between registering multiple handsets and the proxy/remote ip etc. 3cx does not care.
Now the problem comes with registering a remote extension to 3cx in some cases, regardless of how many are registering. So you can go direct or via the 3cx proxy, which you are doing. And the proxy wont care either. It is just forwarding information not really inspecting it.
Now to save time, most phones have a provisioning file which will fill in the same details for you, save you going into each phone. And 3cx creates a file for each phone you ask it to based on a template and it then fills in the information for the extension.
But you can only do this for 1 handset for an extension. It is a limitation in the GUI on 3cx, not for any other reason.
So you can provisions phones manually, or using the 3cx provisioning.
But keep the two things separate in your mind. Registering is one thing. Provisioning is another. Multiple phones can register. Just configure each phone. Provisioning is easy but is limited to one phone per extension (And also one extension per phone).
There are ways around this, but I wont complicate it for you here. Needless to say you can create your own provisioning files if you know what you are doing. But just do it manually on each phone and it will work. I guarantee it.
I have 2 handsets, 3 softphones all registered as my desk extension. And all work with no problem. In fact it is a big selling point to people as it gives them so many options.