If you set up with your back foot even or behind the front of the plate, any forward stride is allowed. IF you are a stationary batter (do not walk the box) that means technically contact could be allowed with both feet infront of the plate if your swing caused your back foot to move forward.
If you walk the box, shuffle, etc, you will be called out if both feet are in front of the plate upon contact.
This is the way I have understood the rule to be written. I am not an umpire but have discussed this with several of the umpires in the area.