The service that we used, Blue Diamond Limos of Texas, didn't require an hourly minimum like many other transportation companies do. Even without that, they still offered us a discount since we rented more than one vehicle. I would imagine that any company would offer some kind of discount if you are using the car/limo for a span of 6 hours. I recommend getting a few quotes (without asking for a discount--see what they quote you) then going back and re-negotiating based off the other quotes you receive. Will you be using the limo for 6 hours non-stop? Will it just be one vehicle?
I need to book mine (I know...procrastination) but mine is located in Southlake. I'm not sure if they will come that way or not (I believe they will) GG limosuine.