There are many methods for carrying a back-up firearm or for deep concealed carry.
One factor to consider when looking at deep concealed carry is access to the firearm.
Most defensive situations happen very fast and individuals have little, if any, warning beforehand.
I typically do not recommend deep concealed carry if the individual is carrying only that firearm.
If a situation were to arise and the firearm needed to defend one’s own life, if that firearm is too deeply concealed on their body then it would make accessing that firearm difficult if not impossible in a timely manner.
The key to concealed carry is to find a firearm that the individual is capable of being accurate with, meets their needs, and to dress around the gun.
There is nothing wrong with carrying a larger firearm; however, that person would need to dress around the gun to properly conceal it.
Deep concealed carry for a back-up firearm is a different story; however, the purpose behind that firearm is different than a primary carry weapon.
As mentioned above, the primary carry weapon should be carried in a manner that will allow the carrier to access the firearm quickly if it is needed for personal defense.
The back-up firearm is often carried in a deep concealed position which makes quick access much more difficult.
The purpose of a back-up firearm is to have a plan B should the primary carry weapon go down for some reason.
That being said, quick access is less of an issue.
One consideration with a back-up firearm is to carry the same caliber as the primary carry weapon due to the fact that if the primary carry weapon were to go down, the ammunition could still be utilized in the back-up weapon.
If carrying a back-up firearm is a consideration, it would be advisable to dedicate some time to training while drawing from the back-up position.