Yes, you can add the tow vehicle to your current passport because you only have the one 10 year permit on it. Technically you could get an entire year for the tow vehicle by changing names. There are a few details, however. You have to have the TIP delivered to an address out of Mexico, or turn one in at the border, then pick up another. Either way, it really doesn't extend your time in Mexico.
If you are going to attempt to turn in the current 10 year permit without going to the border to do it, it is better to attempt it before you sell the vehicle. Remember, there is no guarantee that you will be successful.
