You have to pick:

If this happened, would we, in this universe, even notice a difference?

We'd notice it almost immediately since suddenly there are all these new people wandering around in confusion at how they've been thrust into a parallel universe. :)

I think this would create a rather brutal economic shock. Some areas of the world will see famine and violence (mostly the poor third-world areas that are already on the edge), and even in prosperous first-world areas we'll suddenly have just half the capital resources per-capita that we did before. Having half the population suddenly be homeless and undocumented would be a huge challenge for our various governments. It'd be like everyone just absorbed a massive surge of refugees.

Ooooh. I thought it said "the new people are shunted TO a parallel universe" but it says they are "shunted FROM some parallel universe"