I've been using two Defensive forwards (support) in my 3-4-3 with Hoffenheim. It's actually one of few parts of that tactic that I have got working almost as I want so far. The ball usually gets played to my Attacking forward/Target man in the middle. He then gets to pick which one of the DFs should get the ball. They usually position themselves a little behind the middle man, providing forward runs or a simple passing alternative. Although, sometimes they both go for the forward run, or both stay behind to provide a passing alternative, so I might try setting one of them to be a bit more attacking. Defensively I find that they do a great job of pressing the opponents early on, when my target man can't get a hold of the ball. A lot of times they force the opponents into turning bakc to the keeper, or trying a long pass. Either way, the result is usually a long pass for my defenders and defensive minded midfielders to handle. Which they don't always do very well, so far.
To sum it up, I've had good experience so far playing with two DFs, and a target player type in between. Just have to sort out the rest of the team.
I imagine playing with one DF, he would take a position similair to an attacking minded AM, but perhaps contribute more in pressing the opponents and defending in general. He'd probably be quite close to an opponent alot of the time if you're trying to counter attack, or if you win the ball high up on the pitch. I'd try to make sure the first pass is played to one of the other forwards, who hopefully has a little more space to operate in.