Why do top fuel dragsters have two distributors?

Answer 1

They run two distributors/magnetos so that they can get more spark into the cylinder to help fire the massive fuel amount.Remember that fuel is being forced into the cylinder above the blower,below the blower and through nozzles mounted directly into the cylinder head itself at about 120 PSI.Also MAGNETOS are used instead of regular distributors because the faster you spin a magneto the hotter the spark it makes.But even with all that the fuel charge drowns out the spark completely.That is what happens when the announcer's say a motor dropped a cylinder and it can be seen as a clear whitish cloud pouring out of a exhaust pipe.And if the motor drops enough cylinders the blower belt will break and fly off or the blower itself will backfire and explode into it's restraints.

Answer 2

What he said.
There's 2 spark plugs per cylinder so there's 2 ditributors/Magnetos to ignite the fuel because when the intake valve opens and the air and fuel rush into the cylinder under extreme pressure from the blower it can actually blow out the spark just like blowing out a candle.

A magneto type distributor creates it's own electricity when it spins kind of like a cars alternator. A regular distributor requires a power supply like a car battery to operate.
That's why they use magnetos so that they dont need a battery or alternator etc.....