On the contrary, I think if you disallow the practice entirely, it will be much easier for GM's to determine unruly usage of logging in multiple characters, because all usages would be against the rules.
Cutting down on the amount of reports, by making evading the automatic systems significantly harder, would also make manually report the few remaining offenders much easier. The technical know how to run two clients on two different IP's from the same computer/router is not that trivial and thus only a limited amount of people would both be capable and willing to actually pull it off. Since they would be far and few in between, this essentially makes them stick out a lot more.
Finally, consider this: Back in retail, barely anybody was willing to pay for multiple accounts, we're talking about a fraction of a fraction of a percent of all players here. Servers were lucky to have a single player doing it on them. It is overwhelmingly more blizzlike to disallow the practice than to allow it. The actual advantages that are left if you disallow all the questionable stuff is also really minor. The benefits just aren't there if you remove the overpowered stuff. There really isn't a point to allowing it at all, if you look at the greater picture, the impact on the server and the real tangible benefits you can get from allowing it.