with your time frame you could easily hit up HHN one night and Knott's Haunt the other, and i would highly recommend both as they deliver very different experiences, however, becuase it is the last weekend for the events and Halloween weekend to boot, you can most likely expect the crowds to be VERY heavy and will spend quite a bit of time waiting in line.
my best suggestion would be to get to HHN at least an hour before the gates open, then when they do head immediatly to the lower lot, as everyone else will be filing into the nearest mazes quickly making those lines very long. you hit up the TCM maze and the Terror Tram while in the lower lot, then work your way up to the mazes and shows in the upper lot, which typically get shorter lines towards the end of hte night as everyone who hit them up is now crowding the lines in the lower lot.
for knott's, call and make a reservation for the VIP Pre-scare dinner buffet. That gets you into the park usually 30mins before the general public, sometimes even as much as 45mins before. When you get into the park immediatly go to the re-themed Mine Ride, followed by the re-themed log ride, as those lines tend to be the longest in the park at night.then work your way through all the other mazes and try to fit a show or two in as well.
its a lot to do in one night, but it can be done. my husband and i got tired of trying to rush so much and have made friends with lots of other fans and people who work at Haunt so now we dedicate 6 to 7 nights of our vacation every october to Haunt, and one night to HHN. There are also multiple other haunts around so cal like The Haunted Cellar, Coffin Creek and the new Saw maze that we try to hit up as well, but we're in town for 11 days every year so that gives us a lot more leeway.
hope you get to go to at least HHN and/or Haunt, they are a completely awesome experience