Just beat the game with both Chris and Jill on any difficulty, save after you beat the game, then load that save and go to once again and beat it again with the other character you didn't use the first time on any difficulty.
Note: if you complete both Chris and Jill's story on normal or hard difficulty, you will unlock one dangerous zombie mode, this is not a stand alone mode like real survivor and invisible enemy modes, in ODZ you have Forest Speyer (the zombie sitting outside next to the grenade launcher in Jill's story) following you around in different parts of the mansion, he has grenades strapped to him, he cannot be killed, if you try to kill him he will explode, killing you, this mode is permanent, unless u start over from new game, once unlocked in once again he will appear in very easy, easy, normal and hard, he does not appear in real survivor or invisible enemy modes, he also only appears in the first part of the game, once you go back to the mansion with the hunters, he will be gone, if you want to avoid this mode just simply beat the game with one character on normal or higher, then the other on very easy or easy, this should keep you from unlocking this mode and not having to deal with forest, but really he's not much of a problem, he only shows up in a few parts of the mansion and is easy to avoid.