I'm being warned by my players that one will call for a conflict on the other. Keteb will try to steal Annalise's pocket watch.

How should this be handled? Does one player get to frame everything since they called it? is one Stranger's fate at the mercy of the other's die roll?

MDK

Stealing a watch is not a conflict, it's a task. What's Keteb's goal? Is that goal in conflict with a goal of Annalise? If so (and ONLY then) you have a Stranger vs. Stranger conflict.

Here's how you do it:

First, carefully review the goals of the Strangers to see if they are really in conflict. If they are, then proceed.

The player who did not call for the conflict gets to set the action type and will roll the die. If the Stranger who is rolling has a worse score in the action type than the other Stranger, they take a penalty of 1 to the roll (the range gets smaller by one).

The possible effects of the conflict on both characters should be stated clearly during the “free and clear" stage of discussion, taking both the goal of the acting character and the possible
outcomes along the Series into account.

Regardless of the outcome of the Series, the GM narrates the outcome of the conflict.

Here's an example:

Keteb wants to show Annalise that he's the smoothest Stranger there is. He's going to swipe her pocketwatch to show her just how smooth he is. Annalise's player thinks she is the smoothest Stranger, and won't hear of it. Keteb's player calls for a conflict. Annalise's player gets to set the action type, and will roll for the conflict. Keteb's player thinks it should be Shadow (since it's stealing), but Annalise says that her magical nature makes her aware of all things about her person, so Keteb must contest with Flame. Annalise is better at Flame (big surprise) so she takes no modifier to her roll. Despite her better score, and a re-roll, she fails. The GM narrates. Now, Annalise knows that Keteb is the smoothest Stranger there is. Maybe he still has her pocketwatch, maybe not. That's left to narration since the goal of the conflict was not ownership of the watch.

Nice! You've answered my questions on this topic too.

It's straight out of Trollbabe (which is why it says "Series" instead of "roll" above). I may refine this part of the game a bit. Maybe.