For purposes of time, back/bis, chest/tris are the only times I would ever combine these bodyparts. When doing pushing or pulling movements together the smaller bodypart will suffer. You simply CAN NOT use the same poundages on bis for example after back than if you worked them alone. To get the most out of your arm workouts I suggest doing PUSH/PULL movements. If you have the time the ideal method is doing bis/tris together. My theory is that if your bis and tris get fatigued from indirect movements as is the case when you do Back/bis, Chest/tris you will be limited as to the direct stimulation you can provide to the muscle. Its simple, when you do arms alone you can always lift heavier and hit the area more directly, this is a perfect enviornment for growth and will in my opinion yield the greatest results.