Differential issues

Hello! I am currently building my new Lego McLaren P1. I have not built far yet and have only built the rear differential (or what I think is the differential). However, when I turn one of the wheels, the other spins in the same direction. Is this an issue I should look into? Also, on my old Lamborghini Sian the differentials work but... The gearbox is kind of broken, shifting doesn't work and the car is stuck in place. I hear cogs cracking when trying to move it on the ground. Thoughts?...