you will not have any knees if you do that and that is not a sustainable plan - youre more likely to hate working out and not enjoy it which could make long term goals hard to meet
My understanding is that an insanity workout burns somewhere in the neighborhood of 700-800 calories a pop. If doing that on the course laid out doesn't get you results quickly enough, you're not going to have the patience to stick with any exercise regime.
With an intense workout like Insanity, you really don't need to do it more than once a day. You can actually hurt your results by doing that. Plus, training that much, you increase your chance for injury which would obviously get you no results.