Snakes sneezing sometimes indicate a URI (upper respiratory infection). Sometimes shedding or really dry conditions can cause this, but I'm suspecting a URI. I would look at the temps in the viv and make sure they are OK. I would also see if your dealing with excessive humidity.
Another question that should be asked is if you are noticing any other symptoms, like mouth gaping, swelling around the neck, discharge around the nose, clicking when breathing or back arching?
In the mean time, raise the temps up. Get them in the 88F area and start using a smaller water bowl. Be careful too, because if you have any other snakes, the URI bacteria could be contagious.
Good Luck
Wayne