Fia gt1

I liked the series very much, I'd definetly miss it, but I guess that the Blancpain Endurance series will survive.

Can somebody help me? I dont understand something
I just now started watch these races, and I saw people switching in the middle of the race! is there 2 drivers on one car and is that mean that there is 2 champions? if its true there is another gt races with single driver for each car?
and does its necessary to start your career as a kart driver to get in those races? is this one of the conditions? because I saw that all the drivers started from karts since little age.
Thank you.