Well let me be as straightforward as possible without being blunt. You built two characters, let them take the story where they want to go. let them act with and react to each other. If you'll allow for a modification of the assignment it would go something like this. find a nice starting point (with a hook to capture the readers) then let them play back and forth, they each have goals and desires and those should lead to conflict between them. each has their own method of resolving and or dealing with conflict so show how that works out in a number of minor conflicts then in the major conflict/climax that the story builds too. Hopefully that will be resolved in a satisfying manner which moves at least one character toward a needed change.
So let the protagonist tell the story, let them decide where it leads. You need to work from both points of view, but never both in the same paragraph. If you are unsure where they are headed then the sharp witted back and forth of direct dialog will usually spark the obvious direction. Try your best to let them drive the story rather than make them follow your direction.
It's suppose to be fun....
Mad