Not sure on a non-WoW option for multiboxing.

I see threads for EQ2 and LotRO now and then.

A fair bit have tried Aion, but lately they seem to be viewing boxing as botting.
At least according to a couple of threads.

Warcraft is by far the most boxing friendly environment.
With the "Click to Move" and "Interact With Target", melee boxing is very doable.
And that is quite different, if you want to try something different.