I'm working on my upper body and have singled out chest, flys, biceps, triceps, back, and shoulder. If I wanted to exercise two groups each time I go to the gym, which pairings are best? I have two thoughts. One, it might be best to group muscles that have the most overlap, so chest/flys, biceps/triceps, back/shoulder. Then they would get rest and get worked out hard each time. But, I'm not sure if this would exhaust them too much or be at risk for injury. Two, I would group them with very little overlap, for example chest/bicep, flys/shoulder, tricep/back. Then I would be working separate muscles each time. But, they may not get the rest they need.
A thousand mindless repetitions of an inefficient movement pattern is both a sign of absolute commitment and ignorance.
I understand your question. My answer?
Don't focus your training on inefficient movement patterns by using dumbles and barbells to train each muscle separately. You will lose your ability to move efficiently, and you will create a body that is weak and ragged.
Rather focus on quality movement patterns and try to create a link between LOAD and MOVEMENT. Try to always move in all 3 dimensions. Try to make your training as fun and provocative as possible and avoid a boring routine of 2D exercises.
If you want to explore this kind of exercise and training I strongly advice you to take a look at these websites: