how does twin turbo work..

i was reading up on twin turbos to figure out the point of having 2 turbos, i read that there is a low end turbo and a top end turbo, one that kicks in at low speeds, and the bigger one takes over at higher speeds, but did they mean higher engine revs or higher speed?

There is many types of twin turbo configurations.
There is two stage charging, sequential charging, hyperbar charging and the use of two similar turbos.

Higher speed = engine speed

Originally posted by MaFi0s0
i was reading up on twin turbos to figure out the point of having 2 turbos, i read that there is a low end turbo and a top end turbo, one that kicks in at low speeds, and the bigger one takes over at higher speeds, but did they mean higher engine revs or higher speed?

As SaabJohan said, that's only one type of twin turbo system. It's called a twin sequential turbo setup, and yes the higher speed means higher engine RPM. One small turbo that spools up quickly and provides good low RPM flow, and one larger one that provides the necessary airflow for high RPM boost. The Supra twin turbo is a good example of this type of system.

The other primary reason people use twin turbos instead of a single turbo is for packaging reasons. Larger V bank motors and some flat motors are easier to turbocharge if you have one turbo per cylinder bank, otherwise exhaust routing and length becomes an challenging issue in terms of heat management and performance. Such as system is called a parallel twin turbo setup, where each turbo is identical and the two work together to provide airflow and boost.

In more advanced sequential systems like on Porsche 959, they used valves to control the exhaust and intake flows between the turbos.

Two stage charging is when two turbos is charging in two stages. This mean very high boost pressures without the effectivity losses when using only on turbo. This is used in tractorpulling (dieselengines), pressures up to 17 bar (247 PSI) is used.

Hyperbar charging also uses two turbos. One bigger and one smaller. This system uses a "afterburner".

Another "twin turbo" setup that is currently less common (but becoming more popular) is turbo compounding. With t/c, you typically only have one compressor, but you have two turbines, one that drives the compressor, and one that drives (via gears, fluid couplings, etc) the crankshaft. I suppose that in a weasel-worded sense, that counts as twin turbo, yes?

Turbocompound was invented by Scania and was used on their truck diesel engines. The engines effectivity increased from 44 or 45 to 46%.
Scania used turbocompound until a few years ago, today Volvo is using it in their 500 hp 12 litre engine.

There are other companies currently looking at it as well (not mentioning any names, but several in the US). I thought that specific power output was the biggest incentive, though?

Turbocompound on a Scania 400 hp 12 litre engine:
http://217.31.167.83/~edlund/others/turbocompound.jpg
The file is 596 kB large.