Man, taking benzodiazapines with methadone is not only one of the most intoxicating things you can do but you can die really easy in doing so, especially if you choose to drive. Dentists ( I know for sure, but not for other situations) use Valium ( similar to xanex), and morphine (opiate similar to methadone) for sedation when doing serious oral surgeries. When you mix benzos, and opiates it will heavily sedate you, and will also give you amnesia for the duration of your intoxication.
I say this from experience, I also take methadone for pain ( I was ran over by a car ). When I first started the methadone I almost died because I also have epilepsy and take clonazapam (Klonapin) which is another benzo very similar to xanex. The doctors prescribing the two had now idea I was taking something else. I now take methadone at 6am, and take dilatin at the same time to make sure I don't have seizures during the day, then I take the clonazapam at 10pm to make sure I don't seize while sleeping, and also help me to sleep as one of the symptoms of my epilepsy is severe insomnia.
The doctors make damn sure I'm sticking to this schedule, and not abusing my meds by calling me in randomly to do pill counts and frequently doing drug tests.
So I'm not saying that you should never mix the two, as there are instances where they are needed. But take great caution it's a very, very dangerous concoction. Before I knew this stuff one of the neighborhood kids died from heroin, and xanex, and we all just assumed that since he died with the needle hanging out of his arm that he just took way to much heroin but now I know it was probably the combo that did him in.
As far as being legal to charge you for dui, or dwi you bet it is. I was in court for a speeding violation and a lady was there for the exact same thing and argued what you are, and the Judge explained to her that if an officer deems you intoxicated, by one way or another whether it be by prescription meds, alcohol etc. he is perfectly within his rights to charge you with that offense. So, really if your paying 1500.00 for an Attorney look to double that by the time you pay fines if that charge sticks (in some case with first timers you can get it knocked down to reckless operations). Not to mention your insurance will be greatly effected if convicted if either but I find it very far fetched that a conviction won't result in this matter...