So based solely on the information we have now and the very brief PAX video of the Astrologian's casting animations and sound effects, my theory is that you'll switch stances where one primarily heals (using the Star Globe) while you switch into the other for shorter duration buffs (like a "next 2 attacks" or a 3 - 5m kind) where you use the Divining Deck to buff. So far the only spells that we've seen the Astrologian cast have been cast on players so I have yet to see evidence of any significant debuff ability from the class. Just my two cents.