Well, let's go with the 12 exercises in the All Body XPress workout, since I'm not familiar with the content of the Strength DVD.
So in the All Body XPress, you have:
Single Leg Squat
Balance Lunge
Hamstring Curl
Hip Abduction
Chest Press
Back Row
Y Shoulder Raise
Bicep Curl
Tricep Press
Supine Pull Through
Oblique Leg Raise
Suspended Crunch
So by themselves all done together, it is an awesome workout. But lets say you want to get more out of them. So you'd restructure them in something like this split into two days of workouts:
Workout A
Chest Press | 30 seconds | 40 second transition
Single Leg Squat | 30 seconds per leg | 40 second transition
Bicep Curl | 30 seconds | 40 second transition
Hamstring Curl | 30 seconds | 40 second transition
Supine Pull Through | 30 seconds | 40 second transition
Oblique Leg Raise | 30 seconds per leg (so 1 minute total)
3 sets 1st week - 4 sets 2nd week, 3 minutes between sets
Workout B
Back Row | 30 seconds | 40 second transition
Balance Lunge | 30 seconds per leg | 40 second transition
Hip Abduction | 30 seconds | 40 second transition
Suspended Crunch | 30 seconds | 40 second transition
Y Shoulder Raise | 30 seconds | 40 second transition
Tricep Press | 30 seconds
3 sets 1st week - 4 sets 2nd week, 3 minutes between sets
That breaks up the exercises into intervals, and therefore your heartrate will increase, and afterwards acheive a metabolic reaction.
That would be a way to do HIIT. If you have any questions on that, let me know.
But yes, switching through programs, that would work great. However I would suggest doing the stretching on the Flexibility DVD everyday. You wouldn't believe how tight and tense your muscles really until you stretch them. 