Two people are going on a long trip (let’s say 8 hours). The first guy drives the first 4 hours, and the other one drives the remaining 4 hours. Is it then OK to say they are driving in turn?
Also, are there other “casual”/informal ways of saying/describing that above?
Yes, “driving in turn” makes sense, but it is not the common way to say this. Native speakers will be more likely to say:
They are taking turns driving.