...somebody else than you really are.

It is especially hard if your native language is not English, but hard enough to even native English speakers.

Sooner or later your mannerisms, attitudes and personality traits will reveal you.

It could succeed only if your main nick posts normally and the other nick posts very, very seldom, and his/her messages are well planned beforehand, very thought-through and checked and rechecked again, and then checked again and again on the next day...