Well, technically the recoil never changes. Just the ability for the person to manage the recoil.
Shooting one-handed is often unstable and actually used in many varieties of firearm tests. One of which is the effect limp-wristing has on the firearm, since it's more prevalent when using only one hand. This is why the ATF and other government agencies refrained from using the P250 as their service weapon - too many limp wristing issues which becomes a problem if you're in a fight for your life and the weapon tends to jam when you may not be in the ideal stance (or your primary or off hand is injured/unable to help stabilize the gun).
But, I digress. It is possible that shooting one-handed may have caused Zimmerman to have less control of the weapon. It's possible that the recoil caused the weapon to contact his nose. However, the recoil would have realistically only been able to cause lacerations at best. People that have scopes rammed into their face from improperly shooting high-powered rifles don't often suffer facial fractures, and only minor lacerations above or around the brow area.
:twocents: