im pretty sure the people who put insanity together didnt intend for that. they put it together so month 1 dvds work with month 1 and month 2 dvds work together in month 2. i would keep it the way it is. theirs not really a point of even mixing them... just start over or even just do month 2 and not month 1 if you want to get into the harder stuff.
if you want a challenge, y dont you try
THE INSANITY ASYLUM (click to check it out). and if you want to switch it up y dont you do the p90x and insanity hybrid which would be way more useful to you and your body then a double insanity hybrid..