Bilateral training (that is, lifting with both arms or legs simultaneously) is superior because it allows you to generate greater absolute force.
The amount of combined weight you can lift using one arm (or leg) at a time (unilateral training) is greater than the weight you can lift using both arms together.
A study showed that untrained subjects generated more force contracting one side at a time compared with contracting both sides simultaneously (bilateral force deficit). So the weight you can lift with your right and left sides independently adds up to more than what you can lift with both arms together.
It was later discovered that training history has a significant influence on the degree of bilateral force deficit. In fact, it was found that in experienced weightlifters this deficit was not only reduced, it was also reversed. “Bilateral force facilitation” was observed in experienced weightlifters, whereby they were able to lift more weight using both limbs simultaneously.
Bilateral training allows greater force development in the muscle and thus a greater training stimulus.
For maximum muscle force development use bilateral training. When maximum force is not a priority, unilateral exercises can work well to correct asymmetry.