Convert Front Door Entry Mailbox to a Front & Rear Door Entry Mailbox
What you're going to want to do is this:



That back of the mail box is going to be crimped on in such a way that it can't be removed without cutting the back of the mailbox off.  This would leave sharp edges, so you could round those off and then attempt to re-use the back as a makeshift door, or you could just get another mailbox and use the door from it.  

There's several ways you can use the two mailboxes to make one double-ender, so that bit of creativity is up to you.  I'd probably cut the back 40% off of each one and join the fronts to end up with a slightly longer "limo" mailbox.

I see what you mean about finding a "medium" double door box.  They also have replacement doors that are made for bricked in mailboxes that might work for this, but they seem very expensive for what they are.