Centered stance on a directional twin

I tried a google search and came across the Stepchild FTW snowboard, I hope that's what you meant.  I can't quite find the 2008/2009 Prime model but in any case, I'll try and answer your question.  

So your board is directional, but you wish to center your stance?  May I ask for what purpose?  Since it is traditional camber, you would end up going over the front points of contact with your front foot/binding.  I would imagine this would throw your carves off as you are shortening the distance to your front points of contact and lengthening the distance to your rear points of contact.  If you are hoping to achieve a "twin" setup for park, I recommend you pick up a separate true twin shape board.

A directional twin is twin in shape, but directional in flex. With that in mind, I did a centred stance on a full directional board before (for switch riding) and the results were unhelpful. The issue is that the board's flex profile is meant to be ridden in one direction: stiffer tail, softer nose. I felt it when I ride it backwards at higher speeds since the narrower and stiffer tail (now the nose) likes to continue tracking straight and resist turning. 

I bought my wife a directional twin and I set the board almost centered. the nose is 19 inches long and the tail is 18.5. I talked to the Drake rep and he said that centering a direction board is ok as long as the tail isn't washing out. If it does change the stance to a little set back

^ Yes to spenser's comment.  There is definitely more than one type of directional twin (contrary to what Lux said, no offense).  Nowadays though, it usually does mean twin shape, directional flex, but you should always make sure before buying.  

To add, I think centering stance on a directional camber board would be much more noticeable than if it were a rocker as you don't have two separate contact zones with rocker, just one big one in the "middle".

Unless you're taking it on POW, you won't need to set it back.  I personally love centered stance.  I say keep it there for now, especially for here in Ontario, and enjoy it!

When you hitting up Blue again?

you need to figure out what kind of directional twin you have before deciding where to set the bindings.  here's an example.  let's say yours is the type that is true twin between the contact points, but the nose kick is longer and taller (this is a pretty common type).  if you were to center your bindings based on the nose and tail, you would actually be set forward in relation to the board's edge.  on groomers, that's essentially the same thing as riding a true twin with your bindings set forward.  not really what you want, is it?