I've taken my car into Mexico multiple times from the United States, on a Tourist visa. Is it possible to take more than one car registered to me at the same time (obviously with a second driver)?
From the US Consulate:
So it sounds really like as long as you have the ownership papers and registration, it doesn't matter how many cars in your convoy :D
To add to Mark Mayo's Answer.
There is no limit on the number of cars. If you don't own the car you need a letter from who does. IE if the car is leased you need a letter from the leasing company; If the Bank still owns the car, you need a letter from the bank; etc. The letter needs to say you are authorized to take the vehicle into Mexico. Since you, (the owner), would be with them it should be easier, but you may still want to write the letter for them to keep in the car.
I would refer to Mark's quotation for requirements and add, If you don't have a tourist permit you can not pass checkpoints. The tourist permits can not be obtained at checkpoints. You also need to insure vehicles.
This information comes from Instant Mexico Auto Insurance, they insure vehicles for groups of people involved in international off road racing, and pit stop crews are often driving cars that all belong to the Motor-sporting company.