>>> I've added a second FC HBA to just one of these servers ...
I assume you meant that you added another LUN!? Did this LUN previously contain other data, i.e. a partition table for another partition type/file system? In some cases you may need to clear such "old" partition tables prior to be able to create a datastore from the GUI. In other cases it may work, when you use the vSphere Client and connect to the host directly to create the datastore.
André