Unless, of course, the attacks were supposed to have been simultaneous but didn't come off as planned.
Regardless. Let's say that you have 4 rocks and 100 windows.
You know what's behind the windows.
You pick your windows ahead of time and try and break them all.
There are a lot of better targets than the world trade towers if you're trying to hit politicians.
If you're trying to destabilize a country... you hit 'security' first and foremost.
Where do people look when they talk about their security?
Health, money, law, home (family), gvt.
(Health) Taking out a hospital is a good target. Poisoning a water-supply is better.
(money) Banking and trade centers. The NYSE would've been a good target, the TT was as well (*as a plus.it's highly visible)
(law) Courthouses, police stations, firehalls
(home/family) Attacking a school is a good target.
(GVT) - in your case that'd be White House, Senate, Pentagon, UN building)
Some are easier to hit. Money (the almighty dollar) seems to have been the primary target (remember, they did use 2 'rocks' on one institution..with a hit on the White House and/or Pentagon as icing on the cake.
If GVT was the primary target, there would've been two 'rocks' heading towards the White House, one to Pentagon and one to a secondary target (finance, health, family..pick one).