If they're split (usually are) and I can get in between them quickly, I'll do that instead. Protect and sabotage help me to get in to the center quick so that I'm shooting in both directions.
This is my most preferred approach. I also prefer starting slightly off-center between the two, closer to one of them, which the troops will take out naturally, after the defenses near it are down, based on troop AI prediction, and then I rally against the other.
It all depends upon the layout. Sometimes I will sacrifice a tank, cannon, or general - drop near FC and immediatell rally on it. Then drop other troops near TC and attack like normal. This obviously doesn't work if both TC and FC are well guarded.
This is my second most preferred approach, although it's a variation of this. I start attacking normally, and, after having rallied on the TC, i drop a general or some foot troops near FC, if it isn't well protected, and rely on these troops taking down the nearest target, to take down the FC.
Another approach I like is also to start at one end, destroy the FC, and proceed towards destroying the TC towards the other end - but, I attempt this only if I'm sure of being able to bulldoze my way through the base. If there are anti-tank guns between the two, I don't attempt this.
The 'Banners' chapter at the library, which allows an additional 15 seconds for QV comes in very handy for dealing with bases having Forbidden City (increases the chances of 50% destruction within the first 1 minute and 15 seconds too).
I generally avoid attempting the 50% destruction approach, as I find destroying both FC and TC to be a more accurate plan I can make before the attack, while I find it hard to guess (and plan) the extent of destruction I need to cause in order to achieve 50% destruction.
Some of our allies also prefer to run the Zeppelin (after sabotaging air defenses/towers protecting the FC) to take down the FC. Some prefer to demolish the FC too.
The choice of approach is layout specific to a large extent.
