High Intensity Interval Training?

i am currently on the atkins diet and have so far lost 14pounds, i am looking to change my gym routine such that i incorporate HIIT for 20mins. I am in the gym 5 times a week, i warm up doing 10minutesfast walk, 6minutes quite high jog, 4 minutes cooldown, i repeat this at the end of the workout too...however i have heard that HIIT is more effective for fat burn, so was wondering whether i would be able to do 20minutes of HIIT as at the start of my workout and 20minutes of HIIT at the end of my workout? or would this not be recommended?