They may not object to you sharing, but I think the problem will be that the actual pass they give you is for the number of guests who paid for the Fantasmic dinner package...if 4 guests did the package, then the pass says 4 guests are admitted to the special seating. So if just two of you actually paid for the meals, then I think the pass would show admit two, not three.
I did see a CM turning down people who tried to bring in an extra person on their Fantasmic package pass (near as I could tell it sounded like Grandma joined the family at the park after dinner and they wanted her to be allowed in on their Fantasmic package.)
