Well, what they attempted was to topple one of the towers on the other, which if it had succeeded would have taken out a lot more of the adjacent buildings than actually happened. It would have actually been significantly worse then 9/11. Nobody would have gotten out of the buildings alive, for one thing. And for another, the damage to the other buildings would have been nightmarish.
Fortunately, Yousef was a remarkably incompetent person, just like most people doing this sort of thing. The 9/11 bombers would have probably messed up a more complex plot (one that required managing their own planes, for example).
D
They also had poison gas in their bomb (to kill those in the vicinity/stairwells) except that the bomb incinerated the poison.
It's not like dominos (or the Griswold's at Stonehenge) when you knock down skyscrapers. It's basically impossible to get the center of gravity moved so far that the building will fall over on its side, hitting the next building. I remember in ID4 when the peak of the Empire State Building came off and came down point first onto the street below. But that's the movie, not physics.
Spreading poison is not that easy to do either. You have to do things pretty well, make sure that the airflow carries the poison where you want. A few years ago, there was that nutjob Japanese cult that put sarin, a highly deadly toxin, in the subway, and they only killed a handful of people despite having some well qualified technical people in that terrorist group.