Portals

Question: Can a more combat-savvy Scion hold a victim while his friend opens up the gate to the Oubliette? Similarly, can someone else hold a victim down while the Scion uses Constellation Weaver on him?

Alas, no. Those boons require their users to be able to grapple their victims for a reason, and that reason is that it would be both overpowered and underthematic to do otherwise.

While a badass brawler could certainly hold an enemy down while his friend worked up the mojo for an Oubliette, he wouldn't be able to hurl him in on said friend's behalf; the Oubliette isn't like a wormhole in front of you that anybody can access, but is rather a power accessible to the darkness god - and only the darkness god - who actually controls it. There's nothing for you to throw the guy into unless you, too, can pay for Oubliette and send him launching into the trackless abyss. A god using Oubliette has the ability to hurl his foes away into the depths of unending blackness, but he does not have the ability to let other people who don't have that boon do the same; that power is his and his alone. You can hold that enemy down for him all day if you want to, but when it's actually time to toss him into the dark, the god with Oubliette is on his own.

Similarly, there's no invisible "door" that a god with Constellation Weaver is opening for you to throw a person through, nor is it a power that can be used on folks just standing around on earth minding their own business. The boon represents a star goddess's power to pitch people she doesn't like permanently into the firmament, and if you don't have star powers yourself, you can't do a damned thing to affect the outcome one way or the other.

It sounds like you may be thinking of these powers as opening a "door" that you could throw someone through no matter who you are, or as affecting a person as long as they're held down, regardless of what else is happening. That's not how they work; you would get that effect if you could convince a psychopomp to hold open a Storm the Gates portal into an Overworld, but that's a very specific power that only the masters of travel between worlds have. It's the same story for other powers that allow some gods to interact or communicate between worlds - you can't follow a god using Open Underworld Portal into Hades for free if you don't have the same power, you can't sneak into someone else's Otherworldly Portal for a free teleport and you can't try to hitch a ride on The Milky Road unless a star god expressly brings you along on purpose. These gods aren't really opening doors; they're harnessing their awesome powers over the stars, the blackness, the underworld, or the paths that connect them all. If you don't also have those powers, you can't hope to follow, nor can you force anyone else to.

So if your fellow Scion wants to consign an enemy to the lightless reaches of Keku with Oubliette, she's going to have to do it herself. She's the one with the powers over darkness; she has to be the one who visits them on her victim.

...actually, I take it back. If you're really and truly dedicated to helping someone else use Oubliette on an enemy, I'd say that if you grappled that person and then let the darkness god grapple you and throw both of you forever into the darkness, that might work, pending Storyteller approval. Of course, you'd be stuck hugging an enemy in an oubliette forever, and anyone who retrieved you would also retrieve the enemy, but hey, if you're really, really committed to this, it's a heroic way to go.