Yes, spell effects always stack unless they say otherwise. Sometimes, spells will specify even if they do stack, but this is becoming a rare issue in 8th, since only Sigs can be easily duplicated, and a Wizard may only cast each spell once per turn.
To my knowledge, there is no umbrella ruling saying that a unit cannot be targeted by the same spell more than once, or that they won't stack, so yes, 2x Wyssan casts would be +2, just remember that only one caster will have Throne to make it even better.

 

What does Throne have to do with Wyssan's Wildform?