Generally most developers have no issues with you multi-boxing, so long as you're not using a software or program to automate the process. That is, if you're multiboxing with two or more accounts, and using each account individually, then there's no problem with it (and it's no different, as Deynger said, then me hopping on my GF's account on WoW, PSO2, or XIV for whatever reason).
It's when you're using a software or program that copies keystrokes from one client to another, allowing you to control multiple characters on the same PC at the exact same time, that developers start to take issues.
I'd be curious to see what games not only disallowed this, but somehow made something that valuable not account or character bound in the first place. Otherwise, what you're saying is literally impossible to detect. I mean, how are you going to differentiate me funneling loot I get to my GF for the sake of helping her out, and me funneling loot from an alt account to my main account to gear up my Main. The key answer is - you can't. It's why developers don't take action in multiboxing cases like this - it's literally impossible to tell if something is legitimately gaming the system, or they're simply playing with a family member on the same IP address.
Most developer have also just circumvented this problem entirely by making loot bound to the character that picks it up in the first place.