How to remove dryer from top of stacked washer/dryer in small closet?
Looks to me like there is enough room in there to tilt the whole set over.  Which is likely how the installers did it.  Place some thick blankets/moving pads on your floor.  Make sure you put all the screws back so it is secured and tilt the whole unit over out and out the doorway.  You should then be able to maneuver it enough to get all the attachment screws out and separate the two then til the units back upright.  I strongly recommend getting a second set of hands to do this though.