To my mind string theory is nothing more than fanciful speculation with no supporting evidence (at least none that I know of). Calling it a theory is un-scientific for that very reason.

But even if we assume there is more than one Universe (which in itself is somewhat of an oxymoron} since the word "Universe" implies everything that exists and by definition there can't be more than one "everything that exists" so any other "Universe" would simply be part of the Universe.
But even if there were more than one of what we currently call the Universe at least two would have had to exist before any big bang if this"collision" was to take place. Unless a Universe collided with a singularity or two singularities collided with one another.

LOL this is bordering on the ridiculous and verging on insanity.

However, I believe the real reason why we come up with all these notions of how the Universe might have come into existence is because we can't or won't accept the notion that something could exist in perpetuity i.e. it always existed so it didn't have to be created. And if conservation of energy holds true then this must be the case.

I understand this is a very difficult concept for us to wrap our heads around since everything we're familiar with has a beginning and an end. However this doesn't necessarily apply to the Universe.
According to Occam’s razor the simplest explanation is most probably the correct one and to me the Eternal Universe is much simpler than all the others I've come across.

Even the notion of the collisions of multi verses doesn't in any way address or solve the question of how they might have come into existence in the first place. :)