Clarifying question: what do these two characters want?
Even in a plot-centric story, the big secret to plotting is that the external plot elements ARE character stuff. Every part of the plot should be happening because a character (be it your protagonist, antagonist, etc.) wants something and is trying to get it.
So, you've mentioned two characters here. What strikes me is that the only thing you've told us is what external thing happens to them. But what do these characters want? They had lives before this dubbing-of-chosen ones event happened. What did they think of it? Does one of them want nothing to do with it and immediately does a runner, forcing the other to track him down and conscript him? Is one of them a glory hog with a reckless streak who sees the entire thing as an opportunity to be remembered in song, and so he seeks out every opportunity to do public deeds, regardless of how dangerous or unwise?
We can extrapolate this a bit further. You said that one of them has been dubbed the chosen one even though they aren't actually the real chosen one. Okay, so we have somebody with power who can dub heroes and cares about keeping secrets - what do THEY want? Do they want to ensure that the entire thing goes according to some script, leading them to interfere and stage manage the entire affair? Do they want their chosen one to quietly off the real chosen one and take his place and powers? What will they do to make sure what they want comes to pass?
Going further, you mentioned that the real chosen one's identity is a not only a secret, but on in which if it gets out the REAL CHOSEN ONE will be killed. That's really interesting - normally it's quietly offing the people who know before they can talk, etc. So, who would commit this act, and what do THEY want? Why is it so important to them that the real chosen one act in secret or not at all, to the point that they're willing to throw the entire world they're living in under the bus (including themselves) to ensure it? And what will they do to interfere with the mission to make sure that it plays out according to THEIR script?
So, you've now got four different parties: a dubbed chosen one, a real chosen one who will be murdered if anybody ever finds out what he is, a group that is dubbing a chosen one and managing his quest, and a group that will stop at nothing to keep the real chosen one's identity secret. All of them are going to want different things at different times, and will probably be acting at cross-purposes and getting in each other's way. And that's without taking into account whatever this chosen one has been chosen to fight against (and what does whatever that is want?).
Figure out how they all interact, and you've got your plot.