Have been doing 2/day for a long period of time, back when I and my circle had all started lifting our biggest member commented how my progress has been surpassing all the others eventho I began months later, I could only put my finger on 2/day being my reason. Anyway I prefer doing same group, diff exercise because I am following a 5/3/1 bodybuilding variation. Now, for example if you were doing 2 groups a day and go like, back/chest next day would be what? Legs and shoulders is a no go, because you will have to recover delts from chest and back (anterior and posterior). Arms? Again, chest and back one day before would not equal good arm workout or overtraining. My process is usually AM gym and PM home. 1st session heavy for intensity, 2nd moderate for added volume. Example could be:
1st session, AM, back: Weighted chin up (with respect for our member of high value) followed by barbell work like deadlift, snatch grip deadlift, barbell row, yates row followed by machine work like cable row or other usually followed by farmers walk or a single bicep exercise like cable curl.
2nd session, PM, back and biceps: 2 different kind of pull ups or chin ups not used in AM session, like in this case wide grip pull up and hammer grip chin up or btn or whatever followed by either incline bicep/hammer curl or spider bicep/hammer curl or forearm curls any variation.
Using a program like this hits both intensity and volume but leaves time for resting all groups involved.
Program looks like:
1. Back
2. Chest
3. Legs
4. Shoulders
5. Off
6. Repeat from 1