Best way to get from Barcelona to Carcassonne and back in 48 hrs

Renting a car will probably be your best option. Since you'll be returning to Barcelona, you won't have to pay the stiff fee for picking up a car in one country and dropping it off in another. Driving will be faster than taking the train. And since you would have limited choices for train departure times, a car will give you maximum flexibility so you can make the best use of your 48 hours.

To save money, rent the smallest car that will accommodate the four of you. That won't give you much luggage space, but you won't need to load up for a two-day trip. Take two suitcases and share the space. See the Auto Europe site.

I had a friend that did that trip in One day from Barcelona.... I have no idea how hectic it was but I think 2 days is good enough. He took the train, I think it connects in Narbonne. When I was in Carcassonne I stayed at Best Western Le Donjon Les Remparts. It was inside the city walls and affordable (it's owned by the best western) Clean with nice dining facilities.... the breakfast is worth it. One thing to consider if you take the train, the station is far from the old town so you will have to factor in a cab.

....or bus. The bus takes you right to the gate of the 'Cite'.

We did the train thing as well from Barcelona. Wasn't hectic, but it took several hours, and we had to get off the Spanish train and move to a French train at the border. Don't know if you still have to do that.

We like to spend two nights within the walls of the old city and just 'be' there, enjoying the enoteca's, but a 48 hour trip is doable. That travel time, regardless if you rent a car, which has to be parked outside the city walls, BTW, or taking the train, will still take several hours to do both ways.