We always roll both sails, and we always roll them from the bottom up. That way, you dont have to remove the top battens if it's only for short term storage.
If you are storing the main for some longer time, you should remove all battens anyway to stop them from becoming twisted and then it doesn't matter which way you roll it.
We prefer to roll them separately, as we sail 2-up, easier and faster to rig that way.

Just stuffing the sails into the car and rolling them later isn't something I would have done.

Having a sailbox on your trailer, or secured under your boat really speeds rigging time and it's less hassle to get on the water. Before we got our trailer, we stored the sails at home, and it's so much more convenient to store them close to the boat.