I don't actually like marshmallows enough, so I would probably enjoy two marshmallows less than a single one. Definitely less than twice as much as a single one.
And: given that you are in a psychological experiment, who knows what cruel twists of the experiment the scientists are trying to inflict on you? Perhaps they are actually studying how angry you are going to get when they break their promise? Just eat the damn thing, and they'll have a much harder time taking it away from you.
https://en.m.wikipedia.org/wiki/Stanford_marshmallow_experim...