The only reasonable explanation I can come to, is that you as immortal staff can't differentiate if it's two people or one who is dueling characters.

Introducing technology... Any schmuck can figure out how to use a VPN/proxy and just have his one computer play two characters with two clients. Some clients even have this feature integrated into it to have proxy settings.

There is also the "They can talk in real life" argument. You can also do this with discord, steam, raidcall, teamspeak, skype and so on...

The sad part of it being that anything you can do with a roommate on the same IP, can be done with them being across the world and likely being done by people who don't share IP addresses currently. Just because you can hug trees, doesn't mean the playerbase doesn't sees!

The problem is intent. You can't tell intent and just because it's two people on a shared IP shouldn't make a difference in the matter as long as they aren't obvious. Of course the same IP usually triggers an IMPs attention on the matter, but same IP or not, the cheats regarding same IP can be applied without sharing an IP.