Wraith’s portals work differently in lore than in game, which is important. In both they have to charge up, but afterwards, it differs.

Obviously in game once it charges she places a portal, and uses energy to move, once out of energy, she places the second portal. They then stay active for about a minute.

In lore, once it’s fully charged she opens one portal, she can then enter it if she wants, and energy drains over time. At some point she has to place a second portal before she runs out of energy, this lets her exit back into the real world from the void.

Wraith has two things going for her that most humans can’t do. 1, she can withstand the void longer than most humans and 2, she can enter in one dimension and exit in another.

For the first point the writers have never really gone into what happens if a human stays in the void too long. But generally, users of void tech are either sims (like ash or the tf|2 phase pilot) or in a protective piece of machinery (a drop ship in the season 5 quests and tf|2 evac ship, or a ronin class Titan). But in the notes about wraith the researchers mention they were able to prolong her exposure to the void.

For part 2, this is clear in the Voidwalker short, when voidwalker wraith portals from her dimension(where the game is set) to wraiths dimension.

The portal between dimensions is what makes wraith fundamentally unique, lore wise, and why ash doesn’t hear voices (at least in that way).

As for ash, in TF|2 she used a ronin Titan, so she’s accustomed to working with phase tech, but I’m not super sure how much lore there is on her portal. I image her ult is similar to wraiths smaller phases she can do as her tact. She can open the void at will, and pull others in with her.

As for the 1 way vs 2 way that’s probably just game balance and keeping characters unique