In German the passenger (front right seat) is actually called the "co-driver" (Beifahrer) and there is a cultural expectation to assist the driver with things like reading maps or passing drinks/food or even looking out for directional signs.
There is no such expectation for "guests" on the backseat(s), so this is a pretty distinct role.
This is exactly how my wife and I drive, whichever person is in the passenger seat is the co-driver. We are American, BTW, not sure how common it is for Americans, but I reckon it isn't uncommon.
There is no such expectation for "guests" on the backseat(s), so this is a pretty distinct role.