I want to write the prologue of the story with the narrator in first person with the point of view of the protagonist. But the story is already written in a third person narrator... and I have 2 protagonists so it's not omniscient.
I think it's a cheap trick to change the POV and narrator... I want to write strong "first lines" as a hook and I think it is easiest that way, at least for my idea.
This is not only done, but is a staple of George RR Martin's A Song of Ice and Fire - all books' prologues and epilogues have a one-time POV character that dies by the end of it.
So yeah, it's perfectly acceptable.
If you think it would be a cheap trick, then don't do it.
But it is an already somewhat estabilished tecnique - there are tons of books where the prologue has a different PoV from that of the main characters (I can recall a few at the moment: Perdido street station from Mieville, Eragon from Paolini, Dragons of Autumn Twilight by Weis and Hickman ... ).
Sometimes the prologue is a dream, sometimes it describes something happening in a faraway land, away from the starting point of the protagonist, sometimes it happens in the past or the future.
As others have mentioned, writing a prologue from a different POV than the rest of the story is common enough. The part I'm not sure about is writing the prologue in first person, while the rest of the novel is in third person.
First person feels "closer to the character" than third person. So you'd be making the reader feel closer to a one-time POV that we don't see after the prologue, than you ever let him feel towards the actual protagonists. That feels a bit strange and confusing to me. If you're writing all the novel in third person, I would also write the prologue in third.
As other answers have stated a different POV for prologues is quite a common technique, particularly in SF and Fantasy (George R.R. Martin, Robert Jordan, David Eddings and Brandon Sanderson have all used this to one extent or another)
Changing to first person vs third might be a little clunky though - that said you could "cheat" slightly by having the first person section written in such a way as to make it obvious that it is something written by the POV character rather than it being experienced with them at the time. For example as a letter or a diary entry.
Just to add one little thing to this that the other answers leave out; If you do this, be sure to have a test reader/friend read it and make sure that they are able to follow that a change of POV has happened. As the writer, you know the change in POV has happened, so it will be harder for you to tell if others will also pick up on it without the knowledge ahead of time.
Changing the POV of the story is fine (and is common), but if the flow doesn't immediately provide some clear detail(s) to show that the POV has changed, the reader will become confused as to what is happening, and what the POV is now. With a jump from first two third, when the reader detects a change of POV, their first instinct will be to figure out whose head their in now.
So have others read it, and get their feedback about how jarring/confusing it was for them the first time. (And have new readers each time, as once they know the POV change is coming, they will be biased as well)
Different narratives for the prologue and the story are common. You can definitely try it. In Harry Potter Series, the prologue is from the third person limited perspective of Uncle Vernon, while the whole novel is from Harry's third person limited perspective.
