Why does Boeing 737 use 2 Inertial Reference Systems (IRS) and GPS?
Could somebody explain a little bit why they need 2 separate GPS and 2 separate IRS?


So that if one fails, the other can still be used to complete the flight.

Airbus even have 3.


  What is meant by "alignment"?


Initialization of the system and solving the "heading problem" through a magnetometer. But this is a different question.