For larger engines, say, 2500 c.c. or above, it is better to use 2 smaller turbochargers instead of a big one, as small turbines reduce turbo lag, are easier to spin up and the smaller turbos take up less space.
NB: Correct me if Im wrong but lets presume we have a 3 litre 6 cylinder motor.
Does 1500cc and 3 cylinders spin a small turbo better than a 3000cc 6 cylinder motor spins one twice as big.
The only advantage I can see is that it may make packaging and manifold routing easier.
anyway enough of the doubt on with the theory.
Today, performance cars rarely employ a large single turbo like the early 911 Turbo.
For V-shape and boxer engines, it is also recommended to use twin-turbo, because one turbo serves each bank shortening the turbo pipes and saving a lot of space. Moreover, the shorter the pipes, the less turbo lag generated.
Some twin-turbo engines have the turbos arranged such that exhaust flow from one bank of cylinders drives a turbo which boost the intake of another bank. This is actually the concept of "feedback loop", which helps reaching power balance between two banks.
Most twin-turbo engines have the turbochargers arranged to operate independently, each serves one bank of cylinders. This is so-called "Parallel Twin-Turbo".
An alternative arrangement, "Sequential Twin-Turbo", was designed to improve response and further reduce turbo lag. The turbos operate sequentially, that is, at low speed, all the limited amount of exhaust gas is directed to drive one of the small turbines, leaving another idle. Therefore the first turbine will accelerate quickly. When the exhaust flow reaches sufficient amount to drive both turbos, the second turbo intervenes and helps reaching the maximum boost pressure. Unfortunately, sequential twin-turbo requires very complicated connection of pipes (exhaust from both banks should reach both turbos; so do the intake pipes from both banks), thus it is now losing interest from car makers. Porsche 959, Mazdas 3rd generation RX7, Toyota Supra and Subaru Legacy are the only applicantions left.