Arnold often trained chest and back together, going back and forth between exercises for each. Today we call that agonist/antagonist training.
Antagonist refers to opposing exercises. In other words, an upper back exercise is an antagonist to a chest exercise, and a biceps exercise is an antagonist to a triceps exercise. When creating a program, I like to use exact antagonist exercises.
What in the hell does "exact" mean? Well, for example, if you choose the barbell bench press as your chest exercise for your upper body workout, I recommend a rowing movement with the exact same hand spacing/position as the bench press.
So if your index fingers are 24 inches apart when bench pressing, the rowing movement should consist of a palms-down hand position with exactly 24 inches between your index fingers.
Another example would be with pull-ups (or pulldowns depending on your strength levels). If you do a pull-up with your palms semi-supinated (facing each other) and 18-inch spacing hand position, then your antagonist exercise would consist of standing dumbbell shoulder presses with a semi-supinated hand position that's 18 inches apart throughout the movement.
Got it? This is actually much simpler than it sounds if you think about it. Just remember to press and pull with the exact same hand positions.
Note: This doesn't apply to lower body training. (It's not that it can't be done, it's just more complicated). But what about leg extensions and leg curls? Aren't those perfectly opposing antagonist exercises? Yep, but that particular pairing sucks. In regard to lower body training, just remember to alternate quad-dominant exercises like squats with hip-dominant exercises such as deadlifts.