Well, PotMS only gives it another weapon to fire if it HAS another weapon it could fire.
For example, my Grey Knight Storm Raven has a twin-linked Multi-Melta, a twin-linked Plasma Cannon, 2 Hurricane Bolters (1 on each side) and 4 Mindstrike missiles (of which it can only fire 2 in a turn). That means it has a total of 6 weapons it COULD fire. So if it it is Zooming, with PotMS, it can fire any 5 of those 6 weapons and one can be at a different target to the rest.
Now, let's say that the Storm Raven only has a twin-linked Multi-Melta, a twin-linked Plasma Cannon, and 4 Mindstrike missiles (of which it can only fire 2 in a turn). That means that it has a total of 4 weapons it can fire on any given turn. Now, you can use PotMS to allow one of those weapons to fire at a different target, but you can still only fire 4 weapons, because the Storm Raven only has 4 weapons it CAN fire. PotMS doesn't let you fire one of those weapons twice.