Originally it wasn't that way, but there are some characters who are only allowed as an AU slot (see the Homestuck ancestors due to their severe lack of canon and Hussie's tendency to suddenly dump stuff on us from out of nowhere, or our Majora who wouldn't be appable as an AU because of pretty much just being a mask in the game even if the manga expands on the role). To allow for the potential for two of these characters we decided to allow for two AUs to be in the Tower at once. It's a compromise, really, and if Animus gets big enough that we don't have to worry about casts completely overwhelming the rest of the game I'd like to allow for an OU and two AUs in the future.