I am not currently running 2x a day, but I have in the past run doubles up to 4x a week.

Your plan is basically what I do--a main run once a day, which can be a workout or tempo, then a shorter run the other time.

You say you don't want to run twice just to add miles, but that seems to me a huge benefit not to be underestimated. I also found that I improved my ability to recover by running twice a day. I am firmly in the "miles make a champion" category, though.

Southern Man

I like to run twice a day with easy pace runs. A common combination for me is 6 miles in the morning and 4 miles at night. It's a good way to add mileage and still be able to recover for the next day's run. An easy PM run after an AM tempo run or workout is ok too. I would probably avoid a second run on the long run day, as it may delay recovery. I'd suggest starting out with running twice on one day in the week and adding a second and third day of doubles in following weeks if it still feels ok.