I'm having twins. I installed two rear-faced car seats in my car. But there is no more place for me to sit near them.
Is it nessecary that an adult sit near a newborn baby in his carseat to take care of him?
Nope. If you would be more comfortable keeping an eye on them, there are mirrors you can install that allow you to see them in your rear view mirror. However, you will be wise to watch the road instead of the sleeping babies in the back seat; every newborn I've ever met just sleeps in the car so you won't be missing much.