One Teams license, but 2 email addresses

I have a client who as the title suggests, has a Teams license.

Their 365 licenses are tied to an '@yyynz.onmicrosoft.com' address for licensing. Reason? The account is historically linked to Azure and up until recently they were an independently owned subsidiary of a global player. Therefore they've had their own separate independent subscriptions for 365 and other services. This is set to change as the independent has merged with the global parent. Change is underway, but it's a slow process. 

Using the '@yyynz.onmicrosoft.com' has not been an issue up until now. They've been using the global yyy.com email address for nearly 20 years outside of the Microsoft realm. With the advent of Teams the incumbent account name makes life difficult. Invites are being sent to their '@yyy.com' addresses, which in turn trips up over accessing Teams which is registered to the '@yyynz.onmicrosoft.com' account.

Question I'm trying to get to is, does the client need to invest in additional Teams licenses to accommodate the '@yyy.com' address?  or, and this is the 2nd question, if not, how do I configure Teams to use both?

Bear in mind I cannot accommodate the domain 'yyy.com' in this tenancy as it belongs to the tenancy operated by the global entity. And they're not ready to accommodate these users as yet.

One Teams, two email addresses?

Paul