Shoulders definitely need some iso work, imo. Especially the middle and posterior. I personally don't do any extra anterior work because I feel like I hit them enough on OHP, bench, and incline presses. Improper shoulder development can ruin posture, which gets in the way of everything else in the gym.
Check your form: http://anabolicminds.com/forum/exercise-science/190675-proper-techniques.html
if your gonna do shoulders twice a week maybe do them on chest day after chest then on your other day. i think j86 said thats as well. but dont over train. everyone different. listen to your body. good luck
If you want to train shoulders once more during the week I'd say just focus on the rear delt. Do some snatch grip deads, some heavy partial swings, some facepulls and then maybe some smaller isolation movements like reverse flies or cable stuff to finish.
When it comes to shoulder aesthetics, the rear delts play a huge role. They contribute a great deal to getting that "round" look to the shoulder, not to mention that keeping them strong will prevent a lot of injury and trouble down the road. Looking at the routine you posted above, it looks like you definitely need to ad in some external rotator and upper back work.
It's anecdotal, but for me my shoulders love volume. I'd definitely ad in at least a set or two of something in the higher rep range (12-20).
Also, for a great pump at the end I do this: Use the same pair of dbs for all the exercises and don't rest between movements, just after you've finished all four.
Lateral Raises: 12
Rear Delt Flies: 12
Front Raises: 12
Rest 30 after each round and try and do it 3-4 times. If you like getting a pump that keeps your from touching your head and makes your delts look like little cannonballs, give that a go. I love it as a finisher.
Go hard. Go heavy. Never stop.
I usually hit shoulders once a week. Shoulders are used in bench press (as already pointed out) as well as back workouts to a smaller degree. If you are having problems, mix your routine up. Every once in a while I will go ultra high reps like 50 for failure for several sets. Wonderful burn and will shake it up some.
I train shoulders twice a week. Once with chest and once by themselves.
Day one is chest, which gives my front delts a lot of work anyway. I only do 5-6 sets on overhead press after bench press and incline press. I then concerntrate on side delts and rear delts.
Shoulder day two is rear delt and side delt only- s these are the parts on that are lacking for me. I do 12-16 sets on both rear and side delts (32 total).
From my observations, I think people who have difficulty developing their delts are not performing (or cannot physically perform) a proper over head press. They do not use full ROM, they either cannot bring the weight low enough or cannot lock it out overhead correctly, or have incorrect limb positioning. I think the prior issues are a result of poor scapula control and rotation due to weak serratus and lower traps. Correct your scapula fixators, learn to press correctly, and watch your deltoids increase in size.
Can we stop saying "don't over train". That's such an antiquated warning with little to no consideration for the million things that would come into play. it's not as simple as, "don't train shoulders 3x a week, you'll overtrain". In fact, with most people, you're far more likely to end up with some kind of overuse injury before you reach a state of overreaching or overtraining