If you have been trained to hold your “other strong hand” across your chest when shooting with just one hand, consider this: If you are shooting with one hand, it is likely because your other hand or arm is injured. If that is the case, are you SURE you can hold the injured hand across your chest? If the bones in your arm are broken, that arm will likely be hanging limp at your side. Have you practiced shooting like that? It changes things.
When you are training to shoot one-handed, make sure that at least half of your training includes shooting with the “injured” hand hanging down and out of the way. That arm will likely not be working, so you will need to understand how that changes your balance and movement.
Make Every Round Count
When you go to the range, make sure every round has training value attached to it. Shoot with two hands. Shoot with one hand. Shoot with your weak hand. Don’t just plink. Train.