Again, the currently accepted definitions of protagonist and story goal are insufficient when it comes to accurately describing what is going on within a story. I'm not sure exactly how much of this thread you have read, but I have tried on several occasions to explain that I don't think Laszlo is the most important character in the story (which I assume is your definition of protagonist) or that "Casablanca" is about getting some letters of transit. That is certainly one part of it and the goal that everyone in the story is concerned with, but it is not what the story is primarily about.
To: Bros, two things --
One, forget about loglines. You can't communicate the sum total of your story until you have actually written it.
Secondly, the only way your story will work the way you describe is if there is some other character that fills the Protagonist role for the first half -- and by Protagonist I mean the one pursuing the goal that affects everyone. If you don't have this, the story will linger and plod along with little to no narrative drive. Then, at the midpoint in your story, that character will "hand-off" their dramatic function to your secondary main character and you can finish it the way you originally envisioned.
If you want more help, you can contact me on my site @ http://storyfanatic.com/contact
or you can just PM me here.