Can I move my child car seat to another car?
In theory, yes, but you'll have to check that your child car seat is compatible with the other car you want to put it into. A safe child car seat rests firmly and securely on the car's seat with barely any forwards or sideways movement. Before you head out in the second car, make sure that the seat can be fitted safely well in advance of the trip, carefully following the manufacturer's instructions.
If the child seat's not safe or won't fit into a different car then you shouldn't use it to transport your child in that car - you'll need to find another child seat.