long response!
i recently started doing this and I believe I will be adding a lot of mass with this type of split. I wouldn't recommend it if you haven't been training and understanding how your body responds for a good amount of time, because overtraining is a possibility. The signs of overtraining are allegedly:
1.) faster heart rate first thing in the morning
2.) shaky hands
3.) inability to get a pump
4.) a noted increase in irritability or depressive symptoms
5.) obviously tiredness
the only one i really try to follow is 1.) because it is a concrete way to determine if something physiologically changed in my body. so far so good, i feel fine.
However, I wouldn't be so afraid of overtraining because my personal opinion is with sufficient calories (bulking) and rest, your body is capable of tremendous things and I believe its capacity of training can improve too. I think a navy seal would have a higher overtraining threshold than an average human being no? Undertraining seems to be en vogue nowadays, but to me I think people OVER-worry about that. I have taken my body to the limits of overtraining in my wrestling days, so I know what that's like. dehydration, limited calorie intake, and 4 hour workouts per-day. And believe me, to this day, I haven't had a single session in the gym, during HIIT cardio, or regular cardio that can even hold a candle to any wrestling workout i've had in my life, so I know as far as training goes we can do a LOT before we burnout.
anyway, when using a two-a-day i do something similar to what steveoph says which is alternate a particular bodypart with hypertrophy session vs. strength session
for instance, for legs I may do something in the 6-8 rep range one day, and then I'd hit it again in the 10-15 rep range the next leg workout. For me it doesn't matter if I do two strength sessions in one day, one of each, or two hypertrophy sessions, I just make sure to alternate it for each body part. Obviously each day I try to pick 3-4 bodyparts that are mutually exclusive, and do 2 in the AM, 2 in the PM.
The reason I do hypertrophy and strength is because I tend to get more sore with hypertrophy sessions. In a two-day, you are able to have incredible frequency so being sore all the tiem is counterproductive. I can recover from a strength session within 48 hours if not less, and then bombard it with a hypertrophy session that may take longer than 48 hours to fully heal. I use soreness as an indicator of how to design the split. And yes, there will come a day when soreness becomes nearly non-existent and I will just have to give the body part a full 48 hours of rest by default.
By the way, Lyle Mcdonald has mentioned that the genetic response to training lasts about 36 hours and then dissipates, so he recommends a type of training that allows you to hit the bodypart within that window, stating that once out of that window you begin to go backwards. The standard pound a bodypart once a week doesn't make sense to me since I will never be sore for 7 days unless I just started from a layoff, or I had a ridiculously intense workout (which would lead to overtraining in and of itself).
Also, two a days allow you to have two post workout insulin spikes, which will help you become more anabolic overall.
hard to mention what my split is since it is always changing and won't ever nicely fall on any day schedule, but below is what i'm doing now:
calves and abs will get 3x / week
other major muscles will get around 2-2.5x / week
if say my back heals fast (which it does), I have no problem hitting it 3x a week alternating strength, and hypertrophy as before
all workouts are about 1 hour, if not under
i tend to pick two mutually exclusive bodyparts and hit them in what seems like a superset fashion, but is not. For example, Chest / bicep will be a chest set, sufficient rest, then a bicep set.
