Yes, two licenses are required.
If you only have 2 Exchange servers in your entire Exchange infrastructure, I'd advise against FE/BE structure. In the FE/BE, no mailboxes actually reside on the FE server and it is just used to access BE servers, usually through a firewall. This means if the FE goes down, nobody outside of your firewall can access email at all. It also means that if the BE goes down, ALL email is down.
I always like to have mailboxes for an organization split between two servers. That way if one server goes down, the whole organization is not down. Also, if the down server is unrecoverable (major hardware failure, fire, etc), you can simply create new mailboxes for the users on the good server and restore their old email later.
Your email storage locations also make a big difference. Loss of an email server is not so disastrous if mail is not being stored on the Exchange server (Mail delivered to PST files on desktops or on network shares). If all user mail is being stored on the server (no PST files in use), then loss of that server means the users have nothing unless you restore it.