Wouldn't it be more helpful to create a proc that compares the two users IP and disable a certain function from happening for them? Like say hypothetically I need to get the potion of vengeance and need to sacrifice someone, and I get my brother on for him to create a character to let me sacrifice his character but the IP Check comes in and stops the process which disable me from using Multi Keys to my disadvantage?
Multiple players can legitimately play from the same IP (what if your brother just wanted to play the game at the same time and happened to come across one of the protected functions?), plus you could just as easily ask a friend to do the same thing and completely sidestep the protection.

A perhaps better approach would be to bar new characters from such actions or lessen the reward from them (i.e., maybe sacrificing a level 1 character only gives the weak potion of mild vengeance).