They did mention a bit about possibility of jobs that require multiple class.
You can't be both class at same time to be the job so basically means requiring both class at certain rank to get the job like say..... for example... 10 CON and 10 THM required to get the WHM, BLM. But once you get WHM/BLM job you can be WHM/BLM from both CON or THM class.