What is the proper noun for the guy sitting beside truck driver?

I cannot find the proper noun for the guy who is accompanying the truck driver throughout the journey?Is he considered a truck driver? His responsibility is to drive the truck when the main driver is tired or sleepy.

Also, is there a website or a book that has collection of such jargon?

Solution 1:

co-driver

See, for example, https://www.sciencedirect.com/science/article/pii/S2093791118301070

Whether the truck driver or the co-driver is in charge, or whether they share responsibility,I don’t know. My guess is that one of two drivers makes final decisions about who drives when.

Solution 2:

Handyman: In India, the one who sits beside a truck driver whose responsibility is to assist the driver in driving with a proper nevigation that helps the driver keeping his driving safe is called a handyman.  While, If both are drivers having the same responsibilities and drive alternatively then they may be simply called co- drivers.