Hi Nikk,
According to your description, I suggest you add the second email account as a proxy address. The proxy addresses do not need licenses. Please follow the steps below:
1. Log in to your Exchange admin center and double-click your mailbox.
2. Select email address in the left panel of the User Mailbox window.
3. Click the plus icon to add your proxy addresses.
Here is an image for your reference:
Thanks,
Allen Zhang