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I know that only 2 weapons can be fired on the wraith knight- If I take 2 wraith cannons is there no point putting any other weapons on the shoulders?

This belongs in the Tactics forum.
Adding shoulder weapons would just give him some versatility. You would still only be able to fire two weapons, so it's just a way for you to spend extra points if you've got em'. The main reason they allowed shoulder weapons is so you can twin link the suncannon with a scatter laser, or so you can give him two heavy weapons with the ghostglaive option.

Nope, no point.

Unless you want the cool factor, absolutely no point.

So do you recommend taking a scatter laser and sun cannon or 2 wraith cannons?

most popular loadout is 2 wraithcannons, and then immediately shove him in your opponent's face. He is an absolute hambeast in close combat, the two guns are just gravy on top of that. If you've got the points to spend and need more Ap2 shots, then go with the scatter laser suncannon. This is more effective when you are up against very strong alpha strike armies and just want to avoid your opponent all game and play to objectives.

I opted for suncannon/scatter laser since I play mainly wraith units. This gives me some ranged ability alongside the wraithlords. My guard are currently on foot so I need something to hit from distance. If I find my guard can't compete without serpents, I'dd add a pair but trying to avoid for now.