There are pros and cons to your question. Often in a small town, it is better to at least have a local attorney as representative. Ideally, you should have one attorney that is handling both matter; but if your primary attorney is not from Van Zandt, then have your attorney hire local counsel.
Hire the best DWI attorney you can afford, which is more likely to be in Dallas county. Some attorneys practice in both counties and even if your attorney doesn't, selecting the lead attorney is the first priority. Make sure that your primary attorney is willing and able to assist you by managing or at least having a big say in the costs and strategy of the other county. In selecting your attorney, be sure that the attorney has considerable experience in felony DWI cases-other felonies are too different. Moreover, be sure your attorney has experience working with DWI rehab services, which are stronger in Dallas, but not particularly strong for felonies in either county.