The two weapons must be used very differently. The rear turret is best used from long range. Charge it up and it does a lot of damage, perfect for taking out enemy vehicles and nodes. Keep as close to the edge of the fog as possible and you may not be spotted for a long time. If you can, fire only when no enemy players are facing in your direction to make it more difficult for them to locate you. You will want to be on your own to do this, so don't use the Hellbender for sniping unless you are sure it wouldn't be better used somwhere else. If you see someone else sniping like this do not get in and drive off.
The side turret is best at very close range. The first and most obvious point is that you can perform combos with it. Fire some balls with primary fire them hit one with secondary (it's very easy) to create a chain-reaction that will detonate all the balls you fired, clearing a huge area. It is equally good against infantry and vehicles. Mantas are extremely susceptible, and it usually kills the driver without destroying the vehicle, giving one more Manta for your team to use. It's also very fast at taking out nodes (best to be right next to the node for that so you can alternate between primary and secondary and have it detonate exactly over the node).
If you're driving you must drive such that your passengers (you did pick one or two up didn't you?) can use their weapons to best effect. This may mean you have to stop so that the rear gunner can aim, perhaps to kill an enemy Raptor. To please the side gunner, drive in circles around the enemy so he can fill the area with balls. You need to get close to the enemy for that, so keep moving at high speed to make yourself a more difficult target.
By the way, if you are driving fast with a side gunner, never drive directly at the enemy. You will crash into the skymines the side gunner fires, destroying them and essentially making him useless.
Also, note that the hellbender can be the ultimate defensive weapon. Keep chokepoints flooded with skymines in all different directions (since you can detonate them all in one go, regardless of how far apart they are), switching to the rear turret to snipe anything you see at a distance. And when something comes under your skymines... boom.
The Hellbender makes a great mobile AAA turret, even with just a single driver/gunner. (Use the 1-2-3 keys to move between positions in the vehicle.) For example, in ONS-Dawn, you can park this near a central node, sit in the back turret and fend off the Raptors that might try to come ruin your nodes. With a separate driver/gunner its important for the driver to not dart and weave too much as this makes it nearly impossible to target Raptors.
Against other hellbenders the rear turret works great, but players on foot can destroy a camping hellbender and cause mayhem around him using the shock rifle; the primary shot blasts the skymines of enemy hellbenders effectively blowing them in the shock rifle player favor. This same principle applies using the shock shot from the side turret itself to blow other hellbender's skymines.