There are two ways to do this - one is slightly harder than the other.
The first time I did this activity, it was as such: write a coherent dialogue between two characters - but write only dialogue. You can write no description that isn't in the character's voice, and the dialogue has to sound natural. Easy enough.
Lately, I saw a different version of this: here, the writer-narrator sets up the plot situation in a beginning paragraph - not too lengthy. In this plot situation, the narrator *can not see* but can hear two speakers, one "for" and one "against" the narrator. The two characters discuss 'back and forth' focusing mostly on the narrator rather than on each other. [The speaking characters may be 'developed' as well as the narrator through the dialogue.] The dialogue may have a theme.
So pick one or both and go for it.
I just might try that.. But not here lol. 
