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In the rule book it says
In a turn in which the vehicle neither moves flat out nor uses smoke launchers the vehicle can fire 1 additional weapon at full ballistic skill then normally permitted. In addition this weapon can be fired at a different target subject to normal rules of shooting.
So I read this as saying you can only shoot at a different target if you use the first part of the rule to fire an additional weapon at full ballistic skill. However there is an argument with my group if this is correct as my group is saying that they can shoot at a different target even if they don't have to snap fire.
Example: stormraven shoots lascannon at one night scythe and then uses POTMS to shoot multi melta at other night scythe even though it can fire 4 weapons at full ballistic skill.
Which is the right interpretation RAW.

I tend to go with the HIWPI interpretation of the following:

As long as the target of the weapon you apply POTMS to is different to all other weapons on the vehicle, then whether or not the weapon it's applied to fires at full BS doesn't matter.

For example, I've had my LRC stay still and fire its Twin-Linked Assault Cannon and both Hurricane Bolters at a CSM Squad, and then I used POTMS to target my opponent's LRR (not because I needed POTMS to fire at full BS, but because I wanted to fire at a different target).