Multiclassing shouldn't be a problem this time, I think I've got it working more or less perfect so that you can definitely only be one Career at a time, but feel free to try and break this =D
Unlike the last time, you choose your Career at the start of the game by speaking to that organisation's "Rep" at the school. They will ask you to meet them where you can be given one of two trial tasks depending on your answers in the conversation. Due to the open world atmosphere I'm going for, the third trial task can be received if you skip meeting your Rep and go off on your own. I'm sure there will be lots of scope to break stuff there haha!