Is it bad to workout twice in one day?

Im going to have a strict diet and workout alot this summer to lose some weight, and i was wondering if working out more than once a day would be bad for the body or cause weight gain if i were to stop working out twice a day. I wouldnt do it every day just maybe 3 or 4 times a week do an hour or so of running or zumba incorporated with doing the p90x program everyday. I am also eating healthy foods and watching calorie intake. Will this be a successful 2 months?