OK, lets take a step back here and get a bit more information on your gliders.
I am guessing you have a male and a female. HOW OLD ARE THESE GLIDERS?
I the male intact?
The hiss followed by a short bark is something gliders do to call out to each other. That seems to be very normal.
The "fighting" in the pouch may actually be MATING BEHAVIOR - whether or not your male is neutered, he will try to mate with the female when she is in heat monthly.
When you say you had to separate the - are you putting them in separate cages? or just taking one out of the pouch until they calm down?
Constantly putting them together then separating them into separate cages could be very stressful for them. If they are not actually balling up and fighting then they are probably just working out who is the dominant glider in the cage. You need to keep an eye on them and check for injuries but they will eventually settle things between themselves.
If you have not taken them for a well check up - do so immediately. You need to make sure both are healthy and that there are no parasite issues that one may have shared with the other. If either has parasites - you will need to medicate both.
If the male is intact - have him neutered ASAP. This will calm down the fighting and in general may calm the male down. It will also reduce his scent. Intact males can get very smelly.
If you have an intact male and female pair - You may already have joeys on the way.
You may want to read the information on breeding and raising joeys as well to prepare your self it this is the case.
www.suzsugargliders.com/educationalarticles.htm
Does the 'fighting' seem to be related to the food dish? Be sure to put the food in several dishes and in separate locations in the cage so one glider cannot keep the other from eating.
Try adding or changing the toys in the cage as a distraction. Small toys like monkeys (barrel of monkeys game) toy soldiers, plastic bracelets or even just plain straws put on the top of the cage will keep them busy and entertained at night. They will pull things into the cage to play with and carry around. You may even find toys in their sleeping pouch each morning. A "ball pit" basket with small plastic golf balls, ping pong balls or large craft pom poms can also be entertaining for gliders. Toss a few treats in the ball pit for them to hunt for.