If you ever read anything related to car you might have heard of "Twin-Turbo" or "Bi-Turbo".
What is Twin-Turbo?
Twin-Turbo or Bi-Turbo (Not to be confused with "Twin-Charging") is a set up that utilities two turbos. There are mainly three "Twin-Turbo" set up, Parallel, Sequential, and Compound. All of these turbo set up have different applications. Now let's talk about "Parallel". Parallel is set up where you have two equal sizes turbos spooling at the same time.
Ferrari F154
Parallel Twin-Turbo is mainly used in "V engine". Where you have one turbo per bank. They have equal sizes, they cannot be bigger than eachother. Sequential on the other hand are different they are two staged rather than a single staged like Parallel. Firstly the hot gas exits the manifold to the 1st turbo, then the hot gas exits the 1st turbo to the 2nd turbo. Sequential Turbo can have equal or different sizes. They mainly used in Inline 6, because they can be spooled quicker than Parallel.

Compound Turbo works like a team. Compound Turbo consists of two unequal sizes turbos, one high pressure turbo, and one low preasure turbo. They are two staged like Sequential. First of all, the exhuast gas enters the smaller high preasure turbo or the big low preasure turbo. In this case let's assume it entered the smaller turbo first. The exhuast gas entered the smaller turbo then exits to the bigger turbo. The bigger turbo compressed and sent the air to the smaller turbo to compresses it even more. Compound Turbo is mainly used in Diesel engines.