Then in a sense, is it as good as non-existent?
There are two factors here. The first being that another universe, totally seperate from ours, will have no relation to our passage of time. So you can't really say that it occurs before, or after, or concurrently to our universe, since what we know as time only began with this universe. Is this correct?
The other is the concept of 'nothing', as described by Lawrence Krauss. That is, the idea that 'nothing' doesn't actually exist. We mere mortals cannot comprehend 'nothing', and there is nowhere in the observable universe that consists of 'nothing'. I would take that a step further and say that there is no such thing as non-existence. At least in a literal sense.
I kind of lost my train of thought while I was trying to word this.. Do you guys understand what I'm getting at?