By 50 minutes of high intensity nobody includes the previous 10-15 min of warm-ups. The high intensity times start when you do your first work set not the warm-ups.
I count gym time, not high intensity time. Going by your analysis, in your 50 min workout you're doing more high intensity time than I do during a 2 hour workout.
On any exercise I do, only the last 1 or 2 sets could be considered high intensity. The rest are progressive feel sets (or pump sets to finish with) to get prepared for the near failure, high intensity sets.
In a back workout, if I'm doing for example 20 sets total on 5 different exercises, my high intensity sets would number about 5 at most, so only 25% of that 1 hour workout would be considered high intensity (if rest periods are considered, then even less)...however I would have to be in the gym for at least 75 minutes to accomplish this taking into consideration my warmups. Even my heavy lead up sets at sub-maximal weights could not be considered high intensity, eventhough they can be very taxing nonetheless.
I believe there is too much talk about high intensity here. People aren't training at high intensity for anywhere near as long as they think.
Even in a Bill Star 5x5 type workout, only the last set is high intensity at near-failure weight...and that's only for about 2 or 3 exercises. So what would the high intensity time be in a Bill Star 5x5 workout? A few minutes at the most.
I think people are confusing hard, serious dedicated training with high intensity.
I believe that twice a day training could end up being two half-ass workouts in place of one decent workout for many people. You'd really have to guard again laziness. It would be easy to get into the frame of mind that you can take it a bit easy because you'll be back later to finish off.
I had to laugh when a friend of mine told me he only trained quads (only 3 excises in total) in the morning. I asked him about hamstrings. He said he'll be back in the afternoon to do them...about 2 exercises in total. To me that is sheer laziness apart from being a waste of time.
I train once a week per bodypart in a 4 day split. I know I won't be back for a week working each bodypart so I always have the attitude that I must really blast it or I'm wasting a whole week.
Each to their own. It's once a day for me. If I needed some more time I'd find it more profitable to train on a rest day than come in again on the same day.