you can assign multiple devices to a single extension.
Each device needs a license, so if your idea was to save costs on a license - that won’t work.

but if your idea was - “I don’t want to create another extension, because it is simpler to have both phones under 1 extension” then you can add a 2nd license (i.e. a 2nd digital line) to your user extension. To do so:

Go to users> click on your ext>Devices & numbers > add Phone>pop up opens.

In the pop up you have a step called Select devices. Here you most likely want to select Other phones > “Existing phone”. This makes sense if the 2nd phone is not bought from ringcentral, but from a third party.

Once you add the 2nd digital line head over to 
Phone System > Phones & Devices > User phones
You’ll see both digital lines assigned to your extension. Click on the newly added one and then click on “Set Up and Provision”


 

Thanks! 

When I go to 

Other Phones

Existing Phone

I see select from account (available is 0) and additional purchase then it says (FREE) but when I make that “1” it adds this to the screen to the right as “devices added”

How comfortable with the phone GUI are you?  If you are familiar with going into the phones IP and logging in, you can provision the other line without going through the RC panel. 

The Yealinks have multiple lines that can be added   Once logged into the GUI:
 

change to the Account tab, and use the pull-down to select the next account.

This is where you’ll need the provisioning information that they were tralking about in the previous note.

You can also ask the RC chat about it.

You won’t need all the information - look back at the Account1 settings and you’ll be able to fill out the same fields (mostly) on the Account2
 

Once you have saved the info, near the top it will show ‘Registering’ and then ‘Registered’ when complete.  If it comes back in error, triple check your username/password - that’s where we’ve had the most issues.

This then gives you two lines - same phone - different extensions.  :)

Hope that helps.

That’s exactly right. It means the Yealink device is free and you are only paying for the license.
License or digital line - same thing in my book.

I am going to repeat myself - if your idea was to save on the cost for a 2nd license- that won’t work. 

What Joe is saying is about having multiple accounts on 1 yealink phone. The most common use case scenario for that is having 1 base station with several handsets. 
By default all handsets of a base station would be assigned to account1. 
But if you do what Joe is saying you can assign each handset to a different Yealink account. In other words each handset will have its own phone number and can be called individually. This also requires 1 digital line(license) for each Yealink account. 

My understanding is that you are asking something different. I understood you are using 2 Yealink T57W and you want to assign both of them to your extension. 
To do this you need to follow the steps in my first comment. The result would be that both phones would have a direct number, and by default no matter which number is called your extension would ring. (Similar can be achieved with 2 extensions and 1 call queue, or with 2 extensions and 1 ring group). The default call handling can be changed/ i.e. you can set it up so they ring one after the other, etc.

Example if you don’t want to pay for an extra digital line(license):
- you need something like a Yealink W76P with 2 or more handsets
- all handsets will ring at the same time 
- only 1 license required