So far we have seen 9 of the 10 scions. So for me these are going to be like the ennand a bit where you should defeat 1 before the others. So out of the 9 which one do you consider the most priority target and why and which one you consider the lowest priority?
Will there be more than one in play at any time?
To answer the original question, that could very easily depend on what the back side of those cards do. To TakeWalker, I believe I read somewhere that it would be uncommon for more than one to be out, but there could be two (though they might have to be in different play areas).
Well, Nixious can move play area (when he flips) so it looks entirely possible for more than one to be in the same place, unless there's a specific rule saying otherwise in the Oblivaeon event rules.
I'm going to throw my hat in on what I think each Scion could do on their other side, here goes
Aeon Master: Plays a lot of Aeon Men on his front side, but only to one battle zone, might start doing damage and have damage reducion linked to Aeon Men on his other side, or be completely Minion reliant. I'm guessing that if he flips back, he moves to the other battle zone first, as a retreat and set up elsewhere tactic.
Dark Mind: Steals your stuff and does some damage on the front, but if she flippy floppies (a technical term meaning she'd flip to each side multiple times of the course of a game) she might start discarding cards under her to do more damage on her other side, and flip back when she's out of ammo. (edit) she might discard them to destroy all of them with that name too/instead
Sanction: Has a considerable HP pool, and appears to be charging up for a devastating assault on her flipside. Might play extra Scion cards or discard them to increase her damage on the flipside. Doing damage equal to the amount of cards in the trash seems like a thing to look out for.
Borr The Unstable: He can really rack up tokens on his front, but I don't think he flips back, looks like he might go for an explosive finish. He'll likely use those tokens on his other side, though I've no idea what for...
Empyrion: I've no idea, there doesn't appear to be any hints and I know none of his story (though some might come up during the nemesis episode of the Letters page...)
Nixious The Chosen: does a considerable amount of damage and heals on his front, and on Advanced does not wait around to flip, I don't think I want to see his other side, but taking him down will be hard (because he heals 8HP per turn without any help from the Scion deck) I don't think he'll flip back, unless he flips back when he meets Oblivaeon again. I'm not sure.
(Super Sonic) Progeny: Targets the person with all the rewards, and if he incapacitates them, the rewards might go too, if he flips back, he'll need to get rid of those tokens on his other side, maybe he gets healing on his other side but loses a token when hit?
Voidsoul: Definitely a flippy floppy one, might just flip whenever you flip an objective regardless, maybe he tries to steal rewards, as to flip one reward card needs to exist somewhere...
Faultless: Looks like he only flips once, on his other side he might mess around with even numbers, like "if a hero target would be dealt an odd amount of damage, increase that damage by 1" or maybe he becomes the Faulltless that helps Expat and Setback?
Anyway, these are my guesses, feel free to make your own!