PENUMBRA

Identical twins Feyrn and Teryn step outside of time in refusal to be separated by death. But there, they must watch the murders that started with them, record the tragedies and betrayals that unfold as a handful of powerful wizards destroy everyone and everything that stands in the way of their goal: to destroy magic because it fuels both death and life, and keeps them from those they loved who have passed on. The brothers perform their task, unable to affect anything they witness unless they step back into time.

But, the moment they do, Fey knows Ryn will die--and he would rather see those traitors' scheme succeed than accept that loss.
Until that plotting and manipulating and treachery transforms into something unacceptable, and Ryn can't bear being the reason an entire peoples were wiped out just to ensure no magic rose that could interfere with the one-chance-only leveling of the veil between life and death. There isn't anybody else who can stop the rogue wizards, but the two of them can only do so if they step back into time. So they do. They cut off those traitors' connection to power, ending the capability to plunge the world into a state where life and death aren't distinguished, and nothing truly lives or dies.

Instead, they are separated by the very veil they've saved. Fey still refuses to accept his twin's death, so they petition the Powers that tasked them with watching, seeking only to be together forever, uncaring whether this means they both live, or die, or exist in limbo once again.
And the story moves to another time, when the boys are just beginning to grasp the power that got Ryn killed.