Im done with p90x but have been following a weight training guide 3 days a week at the gym and doing HIIT along with that for 25 minutes. Am I getting the same results doing both of these in the same session than if I did them on separate days?

I don't think there's enough evidence to really answer that with certainty. That's why I tend to go by my own experience.

To me, when I want to do cardio and weight training on the same day (I did this alot prior to P90X) I would either seperate them by as much time as possible, or if I had to do them together I would do weight training first, then HIIT.

Ideally both routines if done to failure will gas you completely, so you could argue doing both back to back is making one or the other suffer in potential. Your body only has so much fuel in its various 'energy systems'. Ask a long distance runner or cyclist and they can explain how they spend and save energy throughout the event and can expend one type but still have another. Steady state is way easier to maintain that heavy weight lifting, or an intense HIIT session. To me, intense HIIT means 1 min breaks between 90% VO2Max segments. I'd love to see someone do that and then do sets of squats/deadlifts anywhere near their 1 rep max.

If I totally burn myself out in a heavy squat/deadlift kind of training session first, my HIIT is going to be more like moderate cardio with very brief bursts of high impact, whereas if I *only* did HIIT it would be really intense. I would say whichever portion is done second is going to be far less intense than it would be if you did it in isolation.

So I guess it depends what your goals are -- fat loss, muscle gain, training for an event? Each would warrant its own answer. For me, I want fat loss along with athletic gains and muscle retention (no loss during fat loss). My answer becomes HIIT in the morning (fasted) and then P90X at night.

Rick

+1... I totally agree with Rick. I have done weight training and cardio during the same session, however, you will need to have enough gas in the tank to do each well. After some time, though, you can get conditioned to do it. Your nutrition is important. I would absolutely not entertain the idea of a low carb diet if you want to weight train and do cardio in one session. I train for triathlons and marathons. I also continue to strength train. Usually, however, I have about 3 hours between running (or swimming or biking) and strength training. There are times, however, when I do them back to back.