Multiple OWA sites, one using Windows Auth, one using FBA and still able to use single namespace?

I have Exchange 2010 SP3 now.  Owa uses Windows Auth only now.

I have Exchange 2013 SP1 installed but with no mailboxes.  I would like to setup this environment so that OWA users on the inside use Windows Auth and external users use FBA.

I've read that to do this I would setup the default website OWA site for Windows Auth, and then setup a second website (with a second IP) with a second OWA virtual directory and set it to FBA.  I would do this with ECP too.  I understand this, but then would I still be able to use the single namespace of mail.doman.com?  Or would I need to set it so that external had mail.domain.com and internal had servername.domain.com?