Yeah, I remember hearing that the towers can only stand because of the other, and that if one went down, the other was bound to collapse. Not sure where I heard that from, but correct me if wrong.
I'm pretty sure you're wrong. I've never heard anything of the sort and can't imagine why that would be the case.
Yousef's plan in 1993 was to knock one tower over and force it to fall into the other, but even if he had a bomb large enough to knock out the supports, that probably wouldn't have worked. Skyscrapers aren't nearly as fragile as people imagine.
Had Flight 11 been the only plane to strike the Trade Center, chances are the North Tower would still have collapsed. (The South Tower would be completely unharmed.) Casualties at the North Tower, IOTL, including passengers on Flight 11, were around 1,700 (most of the casualties on 9/11 were at the North Tower, actually). Due to the fact that you wouldn't have first responders going into the South Tower, you'll be able to save a significantly larger number of people, so you may be able to bump that down as to as low as 1,200 or so.
Even though the South Tower was the second to be hit, it was the first to collapse, mostly because 175 hit at an angle, inflicting more damage vertically as well as horizontally, whereas Flight 11 was flying more or less level when it impacted the North Tower. North being the only one hit means you have less confusion in Manhattan and probably better coordination of emergency personnel.
One would like to think that in that situation the North Tower might even be saved from collapse, but that's probably not the case. Even with a longer span of time to work with, it's not easy to get fire hoses up 90+ stories.
Having re-watched some of the original news footage last year, I took note of the fact that, shortly before the South Tower collapsed, an architectural expert being interviewed by one of the news networks made the comment that the towers would have to be demolished (this, of course, assuming they weren't going to collapse due to the plane impacts).
Can you imagine the massive scale of what a demolition operation would have looked like had the towers not collapsed?
