We've narrowed down the suspects in the couch fiasco to two...
Charlie - newkitty, and prime suspect, though with possible mitigation of fault:
Possible Aliases: Chuck, Trouble, and Scrawny
Max - our first cat, and likely prime motivation for the couch incident, as he's rather a bully sometimes:
Possible Aliases: Maxo, Maxmo, Chubs, and Bucko
It would, however, appear that these two are much like teenaged brothers. When there was only one litterbox, it was a point of territorial contention. Max, being both larger and stronger and the "more established" cat, seemed to cause Charlie enough stress that he'd avoid the catbox and preferred the couch. However, now that there are TWO boxes, they don't have to fight over them...
So they both use either box interchangeably.
Much like their food bowls.
Do not ask me to explain cats.