If you watch the log messages, you'll notice that Dia and Bio can be stacked on certain mobs if Bio is cast first. In this case, you will eventually get messages that both affects have worn off. If, however, you cast Dia, then Bio, you will only get a message about the Bio affect, showing that you canceled Dia with Bio. This does not work on the majority of mobs. It seems that they must have at least a partial darkness tolerance for both to stack.
Edited, Feb 3rd 2009 7:23pm by CaspianFFXI