I think will get better results spreading the workouts throughout the week. Giving your muscles the right amount of time to recover.
Cathe said you can do 2 in a row if you have to because of your schedule, meaning Monday and Tuesday, but to follow an upper day with a lower body day. I have not read anything about combining 2 discs in one day. Also, even the lower body exercises, really recruits your upper body just by holding all those heavy weights. I've read a lot of posts that people are having problems holding the heavy weights to the end of the lower body exercises.
Are you trying to add in extra weight sessions, or do you need to condense all of your weight workouts into a shorter time period?