If you live in Texas and have two cars financed by the same dealership and you haven't kept up with payments on one of them can they take the one that is current on payments?

Read the fine print. Often default on one loan can be cause to call in all loans. If the loan was called, (meaning that you either pay it all off right now or forfeit the car) and you didn't pay off the loan, then they can do exactly what you described.