After watching the newest live letter, it seems to me like the AST can probably be paired with both, or even another AST with ease. The dual stance thing sounds like you can choose to stay in one stance or the other the entire fight. One is based around shields like SCH, and the other around heals like WHM. I think it will be very interesting to see how it plays out, but over the 19th-23rd I will be leveling AST to 50 non stop. I might not even sleep! haha

My hope is that it has very powerful healing potential and very powerful shielding potential but changing between them could be costly. I hope it isn't just a single button to swap like cleric stance.