Questions on Revelation are extremely difficult to answer because they are so highly based on your view of Revelation, which is too hard to argue for in one post. I don't have the expertise to give a great answer to this question, but I thought I might as well give it a shot since no one else has.

As someone who tends towards a [partial preterist](http://hermeneutics.stackexchange.com/a/709/273) position, I would be inclined to say that these were two real men. Who they were, I do not know (though I intend to by [Greg Beale's Revelation commentary](http://www.amazon.com/Revelation-International-Greek-Testament-Eerdmans/dp/080282174X/ref=sr_1_1?ie=UTF8&qid=1336485166&sr=8-1) soon, and perhaps that will shed light on the question).

[John Gill](http://gill.biblecommenter.com/revelation/11.htm) held that they represented a faithful few ministers of Christ down through the ages. He notes that the metaphor of the two olive trees is drawn from [Zechariah 4:3](http://nlt.scripturetext.com/zechariah/4.htm), where they represent Zerubbabel and Jeshua, two faithful witnesses. This interpretation would likely be favored by many [postmillenialists](http://en.wikipedia.org/wiki/Postmillenialism) and [amillenialists](http://en.wikipedia.org/wiki/Amillenialism).

Possible Old Testament parallels to the fire going out of their mouths:

 + Yahweh's word (i.e. what comes out of his mouth) is [like fire](http://bible.cc/jeremiah/23-29.htm).
 + God told Jeremiah that [his words](http://bible.cc/jeremiah/5-14.htm) were like a fire; later, [he complained](http://bible.cc/jeremiah/20-9.htm) that that was so, and that he couldn't hold it in.

For this reason, Gill holds that the fire is the witnesses' faithful speaking of the Word of God.

Sorry for such a bare-bones answer. I will post more if/when I know more.