It was clearly stated that Olivia had the power to open a door between the 2 universes.
It was NOT alternate Olivia for an obvious reason : she has no ability because she didn't follow Walter's experimentations.
In the end, how can alternate Olivia open the door then ?!
Bell and machine were only here to provide power but Olivia opened it.
I love this show but this.... it is a big mistake!!
Seraph of Flame
She didn't. That was the whole point in making a doorstop to keep the door between worlds that Olivia had opened open. Bell provided whatever extra energy that was needed.