Some time ago, I trained 4-6 days per with double splits. In the mid afternoons or morns, (on weekends) I would do my compound work, say BP's, OHP's or Rows etc. then do iso work or arms abs at night. I did not train legs, hips much back then. (pretty dumb I know since this is some of the best body and systemic changing work on those big muscle groups)
After I got into heavy squatting and deadlifting along with my other compounds, I never wanted to train 2x per day, since the leg hip and back work beat me up pretty good so recovery for the next day or so, was always an issue to get ready to hit it again with the big stuff.
Now, I understand your goals are your goals, but from my experience and if you just want to be bigger and stronger than you are now, I am not so sure 2x per day is the absolute best approach, especially if you are natural. Sure nothing wrong with a few sets of arms at night or blowing by a chin bar has you knocking out a few reps, okay I see that. I did and still do that on occasion.
Neither, am I one to go around being afraid of overtraining since I do think that word is strewn about sometimes haphazardly, but truthfully, when I trained 5-6 days per week 2x per day plus worked 40+ hours per week, I had more sore throats and frequent colds and was never any farther ahead on my lifts than when I started doing heavy 3-4 days per week on the 5-7 compounds and working into putting as much weight on the bar as I could on those exercises.
It can be done, but pay attention to your overall recovery and how your feel system wise. It may help in the longer run.
