I was trying to find the litterature on that but I can't seem to find it... There is some good to working out twice/day and I wanted to give you facts...
It would be better if you had two different workouts, walking and biking or walking and weight training, but if the treadmill is all you have that's ok too... I often do 2 workouts/day yesterday it was running and a couple of hours later it was weights ... I also did the same last Sunday and Monday... Running and two different muscle groups of weight training... I enjoy it because it gives me a really good rest and refuel time inbetween the two workouts... You have to remember that working out this often takes time to get used to, I've been working out for 25+ years, over use injury is something that I am very aware of and I often take days off just in case...
Try it though, if it's more convenient for you, by all means do it, I would personnally get gored of 60-70 mins on a treadmill and would much rather do 2 increments of 30 mins each... IMHO...
Just my 2 cents...